Cochrane Collaboration

Background:
Reviews of review-level evidence (tertiary reviews) are desirable when a research question is time-sensitive and/or the scope is broad. However, reviews included in a tertiary review often have only partial overlap with the tertiary review’s research question and, consequently, not all included studies are relevant to the tertiary review. Additionally, the reviews can include some of the same primary studies (known as double-counting). These concerns might lead to biases in the evidence base.

Objectives:
To explore the issues of relevance and double-counting in a tertiary review and to present guidelines for identifying and addressing potential related problems.

Methods:
We examined data from a completed systematic tertiary review on a public health effectiveness topic. We established the relevance of the included reviews by determining how many of the primary studies included in each review met our inclusion criteria and by analysing the included reviews’ synthesis sections using the PICO elements (population, intervention, comparison, outcome) in relation to the tertiary review research question. We demonstrate graphical (plot-based) approaches and a matrix-based approach to establishing the extent of double-counting.

Results:
Of twenty reviews that met our inclusion criteria, ten reviews had less than 50% of the primary studies included that were relevant to our research question. Exploring the synthesis sections of included reviews using a PICO framework was useful in establishing the degree of relevance of the findings to the tertiary review research question and yielded a review ‘utility’ rating. Our graphical and matrix-based approaches allowed us to evaluate the extent of double-counting across reviews; 14 primary studies were included in more than one review, with some studies appearing in four reviews.

Conclusions:
Issues of relevance and double-counting need to be assessed in tertiary reviews, but are often overlooked. The guidelines proposed can help identify potential biases and attempt to address them.

Background:
A Cochrane review about smoking cessation in pregnancy concluded that interventions can reduce the proportion of smokers and the incidence of low birthweight and preterm births (Lumley et al. 2009). Another systematic review (Arblaster and colleagues 1996) suggested a number of intervention features that are promising for reducing inequalities, such as the involvement of peers.

Objectives:
To assess the impact of interventions on smoking cessation in pregnancy for disadvantaged groups, and to evaluate the relative effectiveness of intervention features suggested by Arblaster and colleagues for reducing health inequalities.

Methods:
Various meta-analytic models (meta-regression, multivariate multilevel modeling) assessed the relative impact of interventions on women differing in their socioeconomic status (SES) and ethnicity. The effectiveness of intervention features at reducing inequalities was also explored: using incentives; addressing the expressed concerns of pregnant smokers; involving participants’ peers; and consulting women to design interventions. Some variables were addressed descriptively (e.g., young mothers as a disadvantaged group; structural measures as an intervention feature).

Results:
Interventions reduced the number of women smoking in late pregnancy regardless of SES or ethnicity. Interventions with incentives were more effective than those without; no other intervention features were statistically significant predictors of effectiveness. Interventions were relatively more effective at increasing birthweight than stopping smoking altogether.

Conclusions:
Smoking cessation programmes are effective for all women and should be widely implemented. Analysis is ongoing to identify promising elements for smoking cessation interventions to reduce inequalities.

References:
Lumley J, Chamberlain C, Dowswell T, Oliver S, Oakley L, Watson L. (2009) Interventions for promoting smoking cessation during pregnancy. In: the Cochrane Database of Systematic Reviews, 2009: Issue 3. Art. No.: CD001055. DOI: 10.1002/14651858.CD001055. pub2
Arblaster L. (1996) A systematic review of the effectiveness of health service interventions aimed at reducing inequalities in health. Journal of Health Services Research and Policy 1: 93-103.