1 Introduction
A tremendous amount of data is generated every day, and the use of data becomes more prevalent. Hence, the importance of data literacy rises. The ability to understand data is required now more important than ever to make crucial decisions, including financial, educational, and medical decisions. Data visualizations are often very helpful in making these vast quantities of data more comprehensible and digestible, and there has been a lot of research on the effects of data visualizations on general populations. These works have studied the best uses of color [
63,
69,
71,
78,
87], the best amount of text [
36,
41,
47,
48,
77], and the effects of different chart types [
11,
12,
15].
As the reliance on digital visualizations has increased, data scientists and the visualization community have become increasingly aware of the divide between those who can and cannot access important data via existing visualization methods. For example, the rise in technological advances has contributed to a widening gap in accessibility as people with visual disabilities are unable to interpret increasingly complex data visualizations that new techniques provide [
19,
27].
The question on how to create accessible data visualizations has been the topic of many recent studies. Some researchers explored accessible visualizations for people who are blind or vision impaired [
43], people with intellectual and developmental disabilities [
84,
85], and people with photosensitive epilepsy [
75]. However, little research has been conducted on whether visualizations can be adapted to be accessible for individuals with attention-deficit/hyperactivity disorder (ADHD). ADHD is a neurological disorder that manifests as “impairing levels of inattention, disorganization, and/or hyperactivity-impulsivity”. For many individuals, ADHD may limit effective communication, social participation, or academic achievement [
6,
33]. Because of these traits, the disorder is often linked to inhibiting a person’s ability to digest or analyze information. This is a concern as data-driven decisions become more frequent in people’s everyday lives.
Data visualization guidelines and perception research that apply to a general audience may not be inclusive for people with ADHD, which is a neurodevelopmental disorder. For example, in the context of color, adults with ADHD showed deficits in responding to blue stimuli [
46]. ADHD has also been shown to hinder reading ability, so those with ADHD might be affected by the amount of text used in data visualizations [
21,
60,
61]. The amount of research on accessible data visualizations for ADHD is insufficient considering the prevalence of ADHD today. Rates of diagnosis for ADHD among college students are
\(7.11\%\) in Canada and are ranging from
\(2\%\) to
\(8\%\) in the United States [
53]. Also, approximately
\(2.5\%\) adults around the world were estimated to have ADHD [
73]. Thus, there is a need to further study the effects of ADHD in the fields of computer science and data communication to understand how people with ADHD interpret data visualizations and to provide accessible forms if there are any challenges confronted by those with ADHD.
In this paper, we surveyed existing research related to ADHD and accessible data visualizations. We outlined ways in which the body of work for accessible visualizations can be expanded. We conducted a crowd-sourced survey of 147 participants to test the effect of different chart components – color (hue), text amount, and embellishments/icons – on response time and accuracy for people with and without ADHD. We found that changing these chart components did not significantly affect the responses of those with ADHD compared to the control group. The use of minimal text in graphs correlated with higher performances in both groups. In addition, the responses of those with ADHD to charts using visual embellishments and pictographs were dependent on the task. Based on the findings of this study, we proposed preliminary guidelines on how to make data visualizations more accessible and effective for those with ADHD. We also found evidence that the preferences and personal interests of the viewer did not correlate with performances, but the activation of hyperfocus through enjoyment or stress might need to be considered when designing equitable visualizations. Our main contributions are:
•
We conducted an online crowd-sourced study that included both people with ADHD and people without ADHD to understand how those with ADHD comprehend charts.
•
We found the characteristics of people with ADHD in understanding data visualizations, which are similar to the characteristics of people without ADHD, with respect to specific visualization factors – color, text amount, and embellishment.
•
We suggest design guidelines for data visualizations based on visualization literacy characteristics found in people with ADHD with the goal of helping them better understand their data.
3 Factors in Understanding Charts for People with ADHD
Various types of data are generated every day, and it is necessary to understand such data properly and easily in order to make better decisions. Barriers to accessing data can have a profound effect on a person’s day-to-day life. The goal of creating accessible visualizations is to help reduce these barriers to comprehending data. Understanding the specific ways that people with ADHD see charts could help to create the accessibility of data visualizations and support data-driven decision-making.
To understand how people with ADHD interpret data in visualizations, we discussed our research goals with two domain experts who focus on neurobiological differences in ADHD. They expressed that there is little existing knowledge about ADHD’s interaction with charts, but they confirmed that the goals were worth investigating due to prior research on ADHD’s interaction with vision, reading, and understanding. People with ADHD, in particular, encounter the challenge of balancing attention-grabbing and focus-keeping aspects of charts with the component’s ability to distract or be too stimulating [
29]. Thus, conventional design practices for general audiences may result in inaccessible data communication for adults with ADHD. The experts expressed a desire for empirical research on how chart design decisions affect the chart-reading performance of adults with ADHD.
There are many different components of data visualizations that can influence a chart’s readability. In this study, we selected three chart components: basic color choices, the amount of text, and the use of related visual embellishments or pictographs. Because the choice of color in visualizations influences the efficiency and effectiveness of data perception [
86], we tested basic colors that are commonly used in visualizations. Additionally, based on observations about the amount of text and use of visual embellishments on chart understanding [
77,
84], we tested the effects of the amount of text and the use of related visual embellishments or pictographs, compared to plain charts without any text description or any embellishments, respectively. This study focuses on whether the chart literacy of people with ADHD differs from the general population with respect to specific chart design decisions and, if such a difference exists, how to create more accessible data visualizations based on those findings. In addition to functional differences due to lack of focus, people with ADHD are also known to exhibit extremely strong focus on specific topics or tasks [
7]. This inability to control the object of their focus could cause a divide between design decisions that are functionally effective in boosting readability and the reader’s enjoyment in the chart. Thus, when considering the equality and accessibility of charts, there are two aspects that need to be separated: enjoyment and understanding. This study also highlights whether people with ADHD tend to prefer certain charts and whether that conflicts with their ability to perform tasks or comprehend charts. Domain experts also suggested using an online survey to test our hypotheses since recruiting participants with ADHD who are willing and able to remember or arrive at in-person research labs has become more difficult since the rising use of virtual meetings.
In collaboration with domain experts and based on our literature survey, we designed and tested hypotheses that consist of factors that might impact accessible visualizations for people with ADHD: basic chart colors, the amount of text, the use of related visual embellishments or pictographs, and user preferences.
H1. Chart colors will differently affect response times and accuracy of those with and without ADHD. Colors are used in data visualizations to encode both categorical and numeric values. The perception of color thus affects a person’s ability to comprehend a chart [
57]. However, ADHD is correlated with higher rates of self-reported vision problems, and these vision problems are not represented in physical eye conditions, suggesting that the issue is perceptual or cognitive [
9]. Another experiment also noted that participants with ADHD had more visual problems related to blue-yellow stimuli [
46]. Based on these results, we anticipate that specific hues for charts will target those with ADHD differently.
H2. Increasing the amount of text in data visualization will negatively affect the response times and accuracy of those with ADHD. Several studies have shown that ADHD affects reading ability. Adults with ADHD scored lower in reading speed and accuracy when answering questions [
21,
58,
61]. In addition, people with ADHD took longer times to answer questions based on paragraphs of text than those based on charts [
5]. This is further complicated by the discovery that ADHD is often found alongside other specific learning disabilities, such as a reading disorder commonly known internationally as dyslexia [
6]. We anticipate that the performances of participants with ADHD will have a negative relationship with text amount.
H3. The use of embellishments in charts will improve response times and accuracy of those with ADHD. Researchers have not yet studied how the amount of extra images in charts affects viewers with ADHD specifically. When given distractions that were unrelated to the task of letter search, such as cartoon characters, participants were highly vulnerable to distraction (measured by response speed) [
28]. However, participants’ perceptual load was increased by images that resembled the letters they were searching for, resulting in the level of overall distraction being reduced. For those with ADHD, when individuals are faced with high levels of perceptual load, it can help improve their abilities to focus their attention [
29]. These findings are also reflected when examining Intellectual and Developmental Disabilities (IDD), but the results depended on the specific IDD [
84]. Due to these differences observed among IDDs and because only around 20% of people with IDD have ADHD [
51], we study whether people with ADHD specifically can benefit from different embellishment types. In this study, we use embellishments that represent data, so we expect that the embellishments will help people with ADHD better understand data from charts.
H4. User preferences for people with ADHD will not match the charts that result in the highest performances. When considering the equality and accessibility of charts, there are two aspects that need to be separated: enjoyment and understanding. Little correlation between preference and performance has been found in the domains of musical education [
50], cognitive psychology [
24], and human-computer interaction [
55]. Among participants with ADHD, few were shown to dislike paragraph descriptions of data sets despite the fact that this form led to the longest response times and equally accurate responses to charts [
5]. A negative relationship between perception and understanding has also been found regarding the use of pictographs and icons [
15,
84]. We anticipate that these differences will be repeated in this study, and user preferences for those with ADHD will not correlate with chart components that lead to the best response times or accuracy.
4 Study Design
We designed a crowd-sourced study to confirm our hypotheses, which involved two parts: a problem-solving task to find the differences in completion time and accuracy between groups of participants with and without ADHD and a preference task in which participants ranked charts with various chart components. We asked participants to be as accurate as possible for the problem-solving tasks, and we did not show them the duration time of the study in order to minimize their random guessing.
4.1 Stimuli
In order to create accessible graphs, which are visual forms of data communication, we must understand how people with ADHD interact with graphs in terms of different factors. We designed the study focusing on three factors, which were
color,
text amount, and
embellishment. Figure
1 illustrates examples of the stimuli used in the study.
Color. We generated graphs using different colors. We tested monochromatic ColorBrewer hues since it is one of the common color palettes used in design and academia having some pre-built accessibility options for color blindness [
22,
35]. Color mappings of ColorBrewer colors can also reduce contrast effects [
13,
86]. In digital visualizations,
Gray,
Red,
Blue, and
Green are largely used [
86]. Among them, we focused on testing Red, Blue, and Green colors because digital displays use a combination of these three colors. We used Gray as a baseline color for the response time and accuracy measurements because gray-scale colormaps have been found to be inferior for conveying value information [
82]. For the preference task, we asked for participants’ preferences only on Red, Blue, and Green colors to focus on understanding preference analysis on our target colors.
In the study, we monotonically varied the lightness of each color in a Heatmap (as shown in Figure
1a) to understand how participants interpret color mappings. Using Heatmaps helps to study how participants perceptually and cognitively discriminate values corresponding to the lightness of a color [
69]. In addition, using regularly increasing intervals of lightness in Heatmaps allows us to better control lightness and saturation shown across charts of different monochrome hues [
14]. The intervals of color lightness were predefined by the discrete ColorBrewer scale, which are manually designed palettes in the perceptual color space [
13]. The color palettes follow an evenly spaced sequence of lightness steps, regardless of hue. This ensures perceptual consistency between intervals, making color perception difficulty similar between trials [
13]. Participants were asked to name the coordinate corresponding to the square with the lowest lightness, which would be considered to have the greatest value because people assume that darker colors are associated with larger data values [
69]. A fabricated data set was used in order to lower the chances of knowledge bias and preconceived color associations. In addition, we minimized the chances that participants were simply being drawn to the largest colored area (the area-is-more bias) [
68] by reducing the number of large areas with the same colors. The number of large areas was reduced by using randomized data. Finally, we broke up areas where the same values appear side-by-side with grid markings to also reduce the area-is-more bias [
68].
Text Amount. This study then examined the effect of text on chart comprehension for people with ADHD. We selected four levels of text from the set of stimuli referring to Stokes et al. [
77], which represented increasing amount of labels:
Level 1 (a graph with only labeled axes);
Level 2 (a graph with the axes, title, and one major point labeled);
Level 3 (a graph with the axes, title, and multiple major points labeled); and
Level 4 (a paragraph of text with no chart) (Figure
1b). We chose Level 1 and Level 4 to understand visualization literacy for people with ADHD in two extreme cases: a chart with only labels and only text. Level 2 and Level 3 examined the effect of additional text on charts. Level 1 represents instances when only the chart is used to communicate the data, and only some context to the data is provided in the form of x and y-axis labels. The Level 2 charts added a title and one annotation to the chart. This level served to test annotations related to the main idea of the chart, and charts in level 2 had an average of 21.5 words. Level 3 added more textual annotations and highlighted major trends in the data, such that a large portion of the white background space of the graph was covered in annotations. Participants using Level 3 charts would have to identify which pieces of text are not relevant to the task. These charts had an average of 35.5 words. The Level 4 charts evaluated whether an entirely textual representation of the data without a chart would be best for those with ADHD. Stimuli from Level 4 used an average of 47 words. We labeled the levels such that the value increases as the amount of text used increases.
All stimuli are adapted from the work of Stokes et al. [
77], which used univariate line charts because they are commonly used and are easily annotated. The charts were all generated from synthetic datasets and annotated by a data visualization expert, aiming to emulate realistic but simple graphics. Standard practices, including lightening axis ticks and gridlines, were used in the design of stimulus in order to maintain focus on the line [
77].
Embellishment. We then analyzed how visual embellishments and pictographs affect the chart understanding of people with ADHD. We wanted to examine whether those with ADHD are faster at analyzing less “cluttered” designs or charts with more images, due to the benefit of increasing perceptual load. To do this, we tested three different embellishment types:
plain bar charts,
bar charts with visual embellishments (individual images), and
pictographs (icons) (Figure
1c), as these chart types were examined in Wu et al. [
84] under their visual embellishment experiment. We chose pictographs and charts with visual embellishments because of their ability to engage readers and to improve recall and information finding [
11,
34]. Plain bar charts were gray bar charts with no visuals added to the design. Bar charts with visual embellishments were gray bar charts with a single black image added onto the face of the bars, acting as a representation of the categorical variable. For example, an icon of a chocolate bar was used as a visual metaphor for “hot chocolate”. Pictographs were bar charts in which bars were replaced with stacks of icons representing categorical variables. Each icon represented a set fraction of the value of the overall bar. In order to investigate the min/max and ratio questions, at least more than two categorical variables needed to be represented in our bar charts. Thus, each trial required charts with a minimum of three values. Based on this requirement, we designed that participants view each bar chart with three values to reduce participant frustration and balance the task’s difficulty, following the stimuli creation of Haroz et al. [
34]. We chose to use bar charts for their simplicity in design in order to easily highlight the meaning of the visual embellishments. We also chose bar charts for their parallel similarity to pictographs; such pictographs are most often depicted in bar-like stacks, where the height or width of the stacks represent values [
34].
Currently, customized images created by artists and designers, which are later added to graphs, still fall under the umbrella of visual embellishments [
64]. The embellishments used in this study were created by an artist. All images and icons were relatively simple, consisting of one consistent lightness of gray for line work or to fill the icon. A gray-scale color scheme was used to minimize compounding factors in color choice. We removed the possibility that the participant is distracted by any color choices used to encode meaning.
4.2 Task
Participants performed three tasks. To reduce participants’ frustration and the number of participants who incomplete the task due to the difficulty of the task, we used multiple-choice questions for the text amount and the embellishment tasks. Multiple-choice questions also allowed us to easily assess response time and accuracy. In our preliminary study, we found that the order of task complexity is color (the easiest), amount of text, and embellishments (the most challenging). We repeated the color and amount of text tasks 2 times and the embellishment task 3 times to obtain robust results. At the end of each task, we asked participants to rank their visualization preferences, depicted with the various chart components (e.g., the same chart shown in different colors for the first task). The participants were then asked to share their reasoning in a free-response box.
Color. In the first task, we tested colors in Heatmaps. Similar to a previous color study [
69], our Heatmaps represented a grid of ten zones of a fabricated planet’s ocean crossed with sightings of ten different animal species. The lightness of the color of squares represented the values of the squares. We asked participants to identify the coordinates corresponding to the greatest value in each grid. Each color was tested twice. This resulted in eight color questions (4 colors × 2 repeats).
We chose the task of searching for the greatest value in a graph because it allows us to measure participants’ ability to extract the greatest value, the darkest color, without needing to understand the context. It is also a popular color mapping test [
69,
71]. This task focused on studying how color affects the visualization-reading performance of participants, using the metrics of accuracy and response time. The results of this task can tell us more about whether people with ADHD have different cognitive and perceptual differences to color in the context of graph reading.
Text Amount. In the second task, we showed participants line graphs annotated with various levels of text. We then asked participants to answer a multiple-choice question for trend estimation related to key takeaways from the graph (e.g., “Around which year started the largest increase in immigration?”). We again asked participants to focus on accuracy rather than time to minimize motivation for random guesses. This resulted in a total of eight questions, two questions for each level (4 levels × 2 repeats).
The goal of this task was to have participants examine and understand trend shifts in the line charts/text. We aimed to test trend identification for the time series data because the skill is useful in understanding overall patterns in various time-series data, and it is a common task in time series analysis [
40,
84]. In this task, we studied whether various text amounts aid or hinder the visualization-reading performance of participants.
Embellishment. Finally, for the task testing visual embellishments and pictographs, we asked participants to answer multiple-choice questions on three different types of questions. We used three different types of charts: a plain bar chart, a bar chart with visual embellishments, and a pictograph bar chart. In the study, the participants were asked to answer the following three types of questions for each chart type:
(1)
Search questions asked participants to find the value associated with a category (e.g., “How many cups of coffee were ordered?”)
(2)
Ratio questions asked participants to make judgments of relationships based on values and area sizes of the graphs (e.g., “Which activity receives less than 25% of screentime?”)
(3)
Min/Max questions asked participants to find the largest/smallest value in the charts (e.g., “What type of activity do people spend the most time on while using their phones?”).
Participants were not made aware of the different question types. This section contained 27 questions (3 chart types × 3 question types × 3 repeats).
An array of pictographs can communicate small quantities effectively, as compared to bar charts [
54,
59]. Thus, we chose the Search question to test the value estimation of a specific category for bar charts. An array of pictographs can also be used to represent the relationship between parts and the whole [
15]. To understand how this choice impacts the insights that people with ADHD gather from charts, we used the Ratio question. Additionally, in visualization, locating and reporting specific data is one of the tasks used to measure reader’s understandability [
15]. We chose the Min/Max question to estimate this aspect.
Measuring understanding is a complicated task often replaced by analyzing free-response answers [
15], response time, and accuracy [
11]. In this study, we used response time and accuracy as our objective measurements for understanding. In order to better understand how to create accessible data visualizations for those with ADHD, both preferences and objective measurements were recorded for our tasks. We studied whether there is a significant difference between the preferences of participants with and without ADHD.
4.2.1 Data Generation.
To control the characteristics of the visualizations, we used synthetic data. For the text tasks, we used line charts created by Stokes et al. [
77]. Each graph used an equally complex synthetic data set, ensuring that a difference in responses between trials would be due to a difference in the chart text rather than to the complexity of the data shown. In the color tasks, integers from 0 to 4 were randomly generated for a 10 × 10 grid. One of the coordinates was randomly increased to 5 as the testing coordinate. For the visual embellishment and pictograph tasks, we created data sets inspired by those used in Borgo et al. [
11]. Each bar chart consisted of three categorical variables with small random values (less than 300) since the categorical variables represented everyday physical items such as drink orders. For each data set, the order of categorical variables was rotated between the chart types (plain bar chart, chart with visual embellishments, or pictograph). The values were slightly increased or decreased, but the relative height of the bars to one another stayed the same. This was to help control for visual search based on the categorical variables and actual content, mitigating possible changes to response time based on changing heights of the bars between chart types.
The charts shown when participants were asked to share their preferences were selected to be similar to a real-world visualization. Since the color tasks used the context of a fake alien planet, the charts that were shown to ask for participants’ preferences on color were constructed from the Seaborn “flights” dataset [
83]. The text preference charts had the context of presidential approval ratings, and the embellishment type preference charts had the context of drink orders at a hotel.
4.3 Participants
We recruited a total of 160 participants, through Prolific [
66], 80 each for the group with ADHD (ADHD group) and the group without ADHD (Non-ADHD group). Screening questions were used to ensure that the participants were at least 18 years old, had ADHD for the ADHD group and did not have ADHD for the Non-ADHD group, and were fluent in English. Responses were filtered such that only those with normal or corrected to normal vision, including color-blindness, were included in the experiment. This removed 10 participants with ADHD and 3 participants without ADHD. Overall, we analyzed 70 participants in the group with ADHD and 77 participants in the group without ADHD. Participants were given the title of the survey as well as a short description of its goal and expected tasks. Across both groups, participants took an average of 22 minutes to complete the survey. They were offered an average rate of $8/hr or around $0.14/min as compensation. The ages of all participants ranged from 18 to 54. In the group with ADHD, there were 30 female and 40 male participants (using sex assigned at birth). This showed that the sample population followed the general ADHD population, where more males than females are diagnosed with ADHD [
6]. The largest represented age group was 18 - 24. In the group without ADHD, there were 38 female and 39 male participants. The largest represented age group was also 18 - 24. We also collected the participants’ education level info, which is included in Appendix A (Figure
9).
4.4 Procedure
In order to take part in the survey, participants were grouped by ADHD (ADHD group and Non-ADHD group). The pre-screening was performed through Prolific. Participants were asked whether they have attention deficit disorder (ADD) or attention-deficit / hyperactivity disorder (ADHD) to confirm their eligibility for the study. After giving their consent, participants were asked a series of demographic questions. The tasks were then given in the following order: Color (8 text response questions and preference rankings), Text Amount (8 multiple choice questions and preference rankings), and Visual Embellishments and Pictographs (27 multiple choice questions and preference rankings). The order of tasks was not counterbalanced. To help participants prepare for the more challenging tasks, we ordered them according to their level of difficulty found in our preliminary study, with the hardest task being the last one. Prior to each task section, an instruction page and a sample question and answer were provided. After completing all the tasks, the participants could leave any comments.
We addressed two biases in the study design: the order of questions (Ordering bias), and tiredness (Attention bias) [
11]. Between participants, we randomized the order of questions between tasks. This aimed to help prevent ordering bias, the possibility that answering questions becomes easier after more practice. Randomizing also helps to mitigate the effects of attention bias in our results, in which a participant’s fatigue in doing the same task could affect their responses to the later questions.
4.5 Data Analysis Methodology
Across all three experiments, the response time data was normally distributed after using Tukey’s Ladder of Powers transformation. Additionally, the data had a homogeneity of variances. Thus, we performed Analysis of Variances (ANOVAs) on the data. For the color and text amount experiments, we performed a two-way 2 × 4 ANOVA with two groups (people with and without ADHD) and four factors (color: four colors, text amount: four text levels). For the embellishment experiments, we used a three-way 2 × 3 × 3 ANOVA with two groups (people with and without ADHD), three question types, and three embellishment types. The Tukey p-value adjustment method was used for post-hoc analysis. For our accuracy data, we did not use ANOVA tests since the response variable was either 0 or 1. We created generalized linear mixed models (GLMM) on a binomial distribution. Our experiment used two/three repetitions for each experimental condition. For each result, we computed an average response time and accuracy per subject per condition. Finally, to analyze preference data, we used chi-squared tests and the Bonferroni Adjustment as a post-hoc analysis on the significant results. Since participants were asked to rank their preferences in an ordinal manner, a singular blank response from a participant was filled in with the remaining number. The results from these tests are discussed in the next section.
6 Discussion
In this study, we found that the participants with ADHD completed all tasks faster than those without ADHD. These results may be explained by the activation of hyperfocus, a state of high focus and attention [
7,
32]. It has been studied that those with higher numbers of symptoms related to ADHD also have more frequent hyperfocus events across studying, hobbies, and screen time [
39]. Since participants of this study were told the purpose of the survey, it might have activated their interest or competitiveness, a necessary component for the activation of hyperfocus [
10]. This heightened state of focus can explain why the group with ADHD performed tasks faster than the group without ADHD. It also could account for the similar levels of accuracy between the two groups’ responses, but this result comes as less of a surprise since it has been previously recorded that students with ADHD produce similar quality responses to those without ADHD when answering data-driven test questions [
5]. Despite the findings that those with ADHD were faster than those without ADHD overall, we found evidence that specific chart components did not affect the performances of participants with ADHD differently than those of participants without ADHD. From our results, we created the following preliminary guidelines.
Use similar colors for both people with ADHD and without ADHD: Hypothesis
H1 was
not supported. Best-practice hue design decisions for a general audience may be applied to audiences that include adults with ADHD. Chart colors do not appear to interact differently with symptoms of ADHD. The lack of difference in performance between the groups for the color blue is supported by previous research; several works have found a lack of difference in hue discrimination between participants with and without ADHD [
44,
45]. It was discovered that attention significantly increases the perception of the color blue for both groups, but that does not cause a difference in blue perception between the two groups because those with ADHD have intact covert attention, the ability to pay selective attention to competing stimuli without moving one’s eyes [
44]. The study that found a difference in blue-yellow vision for adults with ADHD explained that a deficiency in dopamine within the central nervous system of those with ADHD may cause differences in the retina [
46]. Since some participants may have been taking medication at the time of our study, the participants may not have had a dopamine deficiency, contributing to why we saw no difference in hue discrimination between the groups.
We discovered many participants in both groups preferred red heatmaps over blue. When asked to provide more explanation for their choices, those with ADHD explained that they were drawn to colors that were more attention-grabbing, such as “red is easier to see, and it makes me pay more attention (P3, ADHD).” Others mentioned the hue’s effect on perceived contrast: “all the hues have good contrast and are readable (P8, ADHD)” and “for some reason, the contrast between the highest and second highest colors is best in red (P11, ADHD).” Likewise, participants without ADHD said, “Red has a bigger contrast, blue is a color that blends well together, and the lighter colors of green are harder for me to distinguish (P12, Non-ADHD)” and “I think that there is more contrast in the red, followed by the blue and then the green which makes it easier to read the data (P4, Non-ADHD).” These similarities in preference may be related to why the groups performed similarly in the hue tasks. Yet, these preferences do not correlate with the hue that led to the best response times in our results, which was green for both groups. Therefore, further work may need to be conducted to understand the relationship between user performance and preferences. In this study, color did not affect the response time or accuracy of participants with ADHD differently from the control group. This is a positive discovery as it opens the number of colors that can be used to encode meaning in charts.
Use a graph with a minimal text annotation: Hypothesis H2 was supported. General guidelines for the amount of text annotation on charts may be applied to audiences that include adults with ADHD. Text annotations on a chart that are not relevant to the task can negatively affect response times and accuracy. Increasing the amount of textual annotations on charts appeared to negatively impact the response time of those with ADHD without significantly increasing their accuracy. In our study, both groups performed significantly faster using the chart with the fewest annotations (Level 1) as opposed to the other text levels (Level 2-4). It was also found that participants had significantly more accurate responses when using the graph with minimal text annotations compared to the paragraph of text.
This is supported by previous research that adding text to a chart can significantly affect the type of information and takeaways that viewers find from the data; it was found that viewers are not likely to take away information that is not included in the annotations [
77]. In this study, each participant was asked to answer one question pertaining to a major takeaway of the charts. Therefore, any text not related to that question may have become irrelevant to the task. We did not always ask a question directly outlined by the labels in the charts. For the paragraph of text (Level 4), many more words were available to act as distractors. This may explain why participants were significantly faster when using the plain line chart (Level 1) than the charts with other text amounts (Level 2-4). It was found that an attention-distractability trait, measured by slow response time, significantly increased with irrelevant visual cues [
28].
This study found evidence that those with and without ADHD perform similarly despite the amount of textual annotations on a chart. Previous work has also found that those with ADHD showed no difference in phonological processing [
52] or in written test-taking response time [
5]. Additionally, the attention-distractability trait was found in general audiences regardless of the severity of ADHD symptoms present in a participant [
28]. Therefore, those without ADHD may be just as susceptible to irrelevant textual distractors as those with ADHD.
In certain cases, text deliberately integrated and placed in the right semantic context can support visual images to improve a viewer’s understanding [
37]. The Level 3 chart was voted as the most preferred chart type by both groups. Like the color preferences, there seemed to be a disconnect between participants’ preferences of text amount and the effectiveness of the design choice (based on accuracy and response time). Participants with ADHD chose the Level 3 chart because “it provides additional context that keeps me interested and makes it easier to remember the data produced (P11, ADHD)” and “it had an extensively detailed amount of info on it to divulge [a] more useful statistic visual (P49, ADHD).” Since the preference charts were not associated with an explicit question to be answered, some found that the highlight of the “maximum” point in the Level 2 chart was irrelevant. One person said it “felt out of place (P54, ADHD)”, and another participant called it “unnecessary information (P20, ADHD).” As seen in these comments, more text may be helpful in understanding the context of the graph, but including extra information unrelated to the question or task at hand impeded the participants, despite their interest in the extra information.
We saw in this study that extraneous text can be similarly distracting for both groups if the task does not directly match the text, causing significant decreases in response time. Therefore, we recommend minimal use of text when designing charts for general audiences, including those that contain people with ADHD, especially if the text is not vital to the message that the designer would like to communicate. This guideline only applies to data visualizations created for audiences with the goal of communicating an idea rather than visualizations created for data exploration. As we can see in the participants’ comments, more textual information on a chart may help when trying to understand a broader view of the data.
Use pictographs for ratio-type and min/max-type questions and plain bar charts for search-based questions: Our findings suggested that hypothesis
H3 is
partially supported. General guidelines for chart type apply to both groups, but it depends greatly on the task types. Pictographs are the best for ratio-type questions and for min/max-type questions; plain bar charts are the best for search-based questions. Although there is a debate on whether the use of embellishments is beneficial, the use of pictographs improved the response times of participants with ADHD in the min/max questions and improved accuracy in the ratio questions. In addition, plain bar charts help those with ADHD understand data in the search questions. Similar to the text-based tasks, an increase of visual stimuli used in a chart may be useful only if the images increase the perceptual load of the viewer; that is, the amount of task-related images should be significant enough to divert attention away from any distracting and irrelevant images [
28]. Evidence of the effect of embellishments and icons on perceptual complexity can be seen in the participants’ comments. One participant with ADHD said the pictographs were “easier to process (P62, ADHD)”, and another stated that charts with visual embellishments were “more engaging (P21, ADHD).” However, there is a balance, as one participant wrote, “Images are helpful and nice, but the little logos are too much and chaotic (P16, ADHD).” In this study, we found a difference in the effectiveness of charts with visual embellishments and pictographs between tasks.
Our search-based questions asked participants to identify and match values with target variables in the questions, and pictographs were the worst embellishment type for these questions. In data visualization, text acts to convey details and mathematical information, whereas visual elements better help viewers to understand the data set’s shape [
49]. For these questions, participants perform their searches mostly based on numerical values, and the added complexity of individual icons in pictographs distracted them from their task [
29]. Additionally, for tasks that require searching for exact values, pictographs tend to cause viewers to count each individual item, dramatically increasing response time [
15,
54]. This inclination to count can be seen in comments left by four participants with ADHD. One participant stated that they were not satisfied with how one icon did not equate to one count of the item (e.g., one cup image in the pictograph represented three drink orders). Another participant said, “[The pictograph] looks too busy and gives me the urge to count the icons to double check and waste my time confirming the statistics (P49, ADHD).” Although we found differences between response times across both groups when participants used plain bar charts and charts with visual embellishments, the difference between these embellishment types for the ADHD group was marginal. This may be because participants do not feel the need to count when they are provided with a single image. Since charts using visual embellishments did not significantly improve response times or accuracy of those with ADHD, we recommend using regular plain bar charts for search-based questions.
We saw a different trend for the ratio and min/max questions. Ratio questions asked participants to estimate the proportions of the categorical variables relative to the whole data set. None of the embellishment types significantly affected response times for the ratio questions; however, pictographs contributed to the highest accuracy overall. It was found that, when using pictographs, people turned to broad estimation tactics when asked to estimate ratios rather than attempting to count the icons [
54]. Additionally, plain bar charts are useful for position and length estimation, not for area estimation [
20]. These observations may be applied to people with ADHD for ratio questions. This may account for why pictographs perform better for these tasks than plain bar charts.
In pictographs, more icons represented larger values, and fewer icons represented smaller values. Thus, it was easier to compare proportions in the ratio questions and to find the largest/smallest value in min/max questions. A possible explanation for why charts with visual embellishments did not improve responses as much despite also acting as a visual metaphor is that they did not aid value estimation in that way. The image’s size or shape did not correlate with the value of the categorical variable, so it merely acted as a distractor [
29]. One participant with ADHD commented, “I like the way [the pictograph] uses cups to symbolize an actual number. The bonus images in [the chart using visual embellishments] just make it more distracting (P8, ADHD).” We see that this participant made a connection between the cups and the value of the variable, but that connection is not seen in the chart using visual embellishments. The images may have helped in min/max tasks more if they were placed at the top of the bar, since their heights would then be mapped to a value in the graph. The effect of placement and size of the visual embellishments needs to be further studied.
The use of icons appears to benefit viewers more than plain bar charts when the task requires more knowledge about the structure of the data, such as in ratio or min/max questions. Plain bar charts appear to benefit viewers more than charts using visual embellishments or icons in simple search-and-find tasks where only textual information, like numerical values, is needed. This leads this guideline on the use of visual embellishments or icons to be dependent on the chart’s goal. Overall, however, those with ADHD did not perform very differently from those with ADHD when comparing embellishment types specifically.
Manage a gap between user preferences and chart performance: Our findings suggested that hypothesis
H4 is
supported. Designers should be cautious and deliberate when using more subjective measures, such as user preference, to influence chart design when creating accessible visualizations for audiences with ADHD. Across the entire study, the visualization preferences of participants with ADHD did not align with the charts that led to the best response times or accuracy. Other prior research has found this disconnect to be related to participants’ preferences for graphs that are familiar [
56]. When studying why people make certain color decisions for graphs, semantic associations, which depend on cultural context, and bias were found to affect reasoning [
2]. Studies also found a relationship between dislike for a chart and how much time a participant perceives is needed to understand and respond to a chart, regardless of actual performance [
15,
84].
A gap between user preferences and chart performance for people with ADHD may also be explained by the priming factors of hyperfocus. Hyperfocus has been found to be activated by both enjoyment [
7] and by stress [
32]. It is possible that for certain tasks in this survey, participants with ADHD were motivated by stress, which would lead to their high performance as well as their dislike of the charts. Although they enjoyed certain other chart components, they may not have been as motivated to answer the questions correctly while in a more relaxed state. Therefore, understanding the preferences of a viewer with ADHD may be important in increasing engagement with data visualizations and lead to hyperfocus status by enjoyment, not stress.
This study also showed that chart preference appears to be a subjective factor. We saw many contrasting comments on preferences between participants with ADHD. One participant with ADHD said, “The red chart seems to be more easily readable (P1, ADHD)”, but another said, “Red is hard to read (P50, ADHD).” One participant called the Level 2 chart the “best visualization” despite choosing the Level 3 chart as their favorite because it “had the most information (P6, ADHD).” A split between preference and effectiveness of the chart can also be seen when one participant with ADHD commented, “[the pictograph] has too much going on, it’s interesting but not intuitive (P40, ADHD).”
Charts with visual embellishments were cited as being helpful in reinforcing the text. One participant described the charts with visual embellishments as the “fastest to read (P20, ADHD)”. Another participant with ADHD agreed that they were “more engaging as it contains visualization of [variable types]”, whereas the pictographs were “unnecessarily complicated (P21, ADHD)”. This was not reflected in the response times of this study. Many of the participants (eleven participants with ADHD) used the term “distracting” as a reason for why they did not like either the chart with visual embellishments or the pictograph. Some of the participants (nine participants with ADHD) said “less was more” and “simplicity is best.” From these results, it appears that a study focus of just ADHD did yield different results from those of Wu et al. [
84], which found participants with intellectual and developmental disabilities (IDD) showed a greater inclination towards icons as compared to those without IDD. In our study, fewer participants with ADHD preferred the pictographs over those without ADHD. This identifies a need to differentiate between ADHD and IDD in data visualization design. Designers aiming to create visualizations specifically made to address an audience of people with ADHD should carefully consider a balance between their stated preferences and the goals of the chart.
6.1 Limitations & Future Work
Our work only covered a few chart components that can be adjusted to create more accessible visualizations – color, text amount, and types of additional embellishments. Future work may study the effect of blurring distracting chart features and animation on audiences with ADHD. These features have been studied before with the goal of improving educational tools for children who have ADHD [
8]. For the color tasks, many other variables could be explored. In this study, we focused on how hues affect response time and accuracy without varying the size of the markings. However, it has been shown that the size of the marking affects color perception in a general audience [
78]. The interacting effect of chart marking sizes and colors on audiences with ADHD should be examined. Other color palettes outside of those belonging to ColorBrewer could also be examined for their accessibility. There is little research on the relationship between color and the interpretability of a line chart, especially with the added context of ADHD. Therefore, future studies should investigate whether colors used in the text amount task or the embellishment tasks may have had interacting effects with our results. Future research should also expand upon this survey to investigate how color, text amount, and embellishment type interact with ADHD in the context of other chart types and encoding choices. Similarly, future work may investigate how chart components interact with ADHD in the context of other task goals.
There are limitations to this study that could be later examined. In all three tasks, the similarity of accuracy and response time between the groups may be explained by the competitive and goal-oriented nature of the survey. This is something that is not always seen in real-world visualizations, such as when a viewer encounters a chart passively online. The effects of “hyperfocus” and the study of how and whether to activate such a state in those with ADHD should be considered in the context of data visualizations. This study used a crowd-sourcing service and an online survey, and in order to increase access to the survey, computers, phones, and tablets were all allowed as testing equipment. Thus, there was no guarantee that participants viewed the stimuli at the intended size. The brightness of the screens may also have been variable and have affected the color-identifying tasks. A controlled setting in a lab could be created now based on the results of this study. The use of eye-tracking software in order to better understand the preferences and responses could then also be used.
In addition, it should be acknowledged that since we used an online survey, we could not control the relative time that the participants took the survey. This is especially the case if respondents lived in several different time zones. This may have a slight interacting effect with our results since it was found that learning later in the day is especially improved for those with ADHD [
72]. However, since the time at which a viewer interacts with a data visualization is not something that can be easily controlled in real-world situations, the effects of controlling the time of day at which experiments are conducted will need to be further contemplated.