Case-Control Studies
- 1. Case-Control Studies Dr. Faiza Abou El-Soud Prof. of Community Health Nursing, Menuofiya University ā Egypt
- 2. ā¢ To describe the design of case-control studies. ā¢ To discuss potential biases in case-control studies ā¢ To differentiate between cohort and case-control studies ā¢ To analyze case-control studies ā¢ Discuss strength and weakness od case-control studies
- 5. ā¢ History of medicine has always been fascinated in discovering the causes of the disease and the ways in which these could be modified.
- 6. ā¢ Investigator does not assign the exposure ā¢ Makes careful measurement of patterns of exposure and disease in populations ā¢ Comparison group ā¢ Make inferences about exposure and disease
- 7. ā¢ The observational epidemiologic study of persons with the disease (or other outcome variable) of interest and a suitable control (comparison/ reference) group of persons without the disease.
- 8. It involves two populations ā cases and controls and has three distinct features : ā¢ Both exposure and outcome have occurred before the start of the study. ā¢ The study proceeds backwards from effect to cause. ā¢ It uses a control or comparison group to support or refute an inference.
- 10. 1. Selection of cases 2. Selection of control 3. Information on exposure 4. Analysis
- 11. ā¢ All people in source population who develop the disease of interest āSample of cases āIndependent of the exposure under study ā¢ Clear definition of outcome studied ā¢ Prevalent . incident cases āPrevalent cases may be related more to survival with disease than to development of disease.
- 12. ā¢ Hospital/clinic based cases āEasier to find āMay represent severe cases ā¢ Population based (cancer registry) ā Not biased by factors drawing a patient to a particular hospital
- 13. ā¢Represent the distribution of exposure in the source population of cases āSelected from the same source population that gives rise to the cases ā¢ Selected independently of their exposure status.
- 14. Population based Sampling of the general population Health care facility based Patients with other diseases Caseābased Friends, Neighbourhood
- 15. Matching Defined as : ā the process we select controls in such a way that they are similar to cases with regard to certain pertinent selected variable which are known to influence the outcome of disease and which if not adequately for comparability could distort or confound the result ā.
- 16. Types of Matching Type 1 assigning cases to subcategories based on their characteristics like age occupation, etc. and then establishing appropriate controls. Type 2 It is finding a control for particular case as closely resembling as possible except for disease under
- 17. Selecting good data on exposure 1.Objectively ā¢ Reproducibility of exposure measurement 2. Accurately ā¢ Information reflecting as closely as possible the effect of exposure 3.Precisely ā¢ Quality management in exposure measurement
- 18. Selecting good data on exposure 1.Objectively ā¢ Reproducibility of exposure measurement 2. Accurately ā¢ Information reflecting as closely as possible the effect of exposure 3.Precisely ā¢ Quality management in exposure measurement
- 19. 3. Information on exposure āPersonal interviews / mailed questionnaire āReviews of records e.g., Dose of drug, radiation, type of surgery etc. āMedical examination or special test e.g.,Blood pressure, serum cholesterol āEnvironmental survey e.g.,Water, Air, Sanitation status etc.
- 20. 4. Analysis
- 21. 4. Analysis
- 22. Interpreting Odds Ratio ā¢ ā¢ OR = 1 -Odds of exposure among cases and controls are same - Exposure is not associated with disease ā¢ ā¢ OR > 1 -Odds of exposure among cases are higher than controls - Exposure is positively associated with disease ā¢ ā¢ OR < 1 - Odds of exposure among cases are lower than controls - Exposure is negatively associated with disease
- 23. Example:
- 24. Answer
- 25. Answer Odds ratio = (100 Ć 7,920) ā ( 80 x 1,900) = 5.2 ā¢Interpretation ā¢ ā¢ OR > 1 -Odds of exposure among cases are higher than controls -Exposure is positively associated with disease
- 26. Types of Bias in Case Control Studies ā¢ Bias ā is a systematic error in design, conduct or analysis of a study which an erroneous conclusion. ā¢ 1.Bias in selection of cases - selection bias or diagnostic bias ā¢ 2.Bias in investigating controls ā¢ - , the controls are less likely to recall exposure variables than the cases. ā¢ - that may lack depth in controls whereas the cases are thoroughly worked up
- 27. Types of Bias in Case Control Studies ā¢ 3. Confounding Bias ā¢ Distortion of study effect with another effect because of variables to the exposure affecting the prediction of the disease) ā¢ When the disease has multiple risk factors which are related to each other ā¢ ā MATCHING BETWEEN CASES AND CONTROLS ā¢ 4.Bias in analysis ā¢ - the presence of a confounder is mostly identified at the time of analysis. ā¢ - It is due to non- uniform distribution of confounders. ā¢ ā Stratification ( limit the size of study and no of confounding factors)
- 28. Strengths in Case Control Studies ā¢ Good for examining rare outcomes or outcomes with long latency, ā¢ Relatively quick to conduct, ā¢ Inexpensive. ā¢ Requires comparatively few subjects ā¢Multiple exposures or risk factors can be examined.
- 29. Weakness in Case Control Studies ā¢ Susceptible to recall bias ā¢ Selection of an appropriate comparison group may be difficult ā¢Rates of disease in exposed & unexposed individuals cannot be determined
- 31. Thank You