Abstract
BackgroundAn important part of the systematic review methodology is appraisal of the risk of bias in included studies. Cochrane systematic reviews are considered golden standard regarding systematic review methodology, but Cochrane’s instructions for assessing risk of attrition bias are vague, which may lead to inconsistencies in authors’ assessments. The aim of this study was to analyze consistency of judgments and support for judgments of attrition bias in Cochrane reviews of interventions published in the Cochrane Database of Systematic Reviews (CDSR).MethodsWe analyzed Cochrane reviews published from July 2015 to June 2016 in the CDSR. We extracted data on number of included trials, judgment of attrition risk of bias for each included trial (low, unclear or high) and accompanying support for the judgment (supporting explanation). We also assessed how many Cochrane reviews had different judgments for the same supporting explanations.ResultsIn the main analysis we included 10,292 judgments and supporting explanations for attrition bias from 729 Cochrane reviews. We categorized supporting explanations for those judgments into four categories and we found that most of the supporting explanations were unclear. Numerical indicators for percent of attrition, as well as statistics related to attrition were judged very differently. One third of Cochrane review authors had more than one category of supporting explanation; some had up to four different categories. Inconsistencies were found even with the number of judgments, names of risk of bias domains and different judgments for the same supporting explanations in the same Cochrane review.ConclusionWe found very high inconsistency in methods of appraising risk of attrition bias in recent Cochrane reviews. Systematic review authors need clear guidance about different categories they should assess and judgments for those explanations. Clear instructions about appraising risk of attrition bias will improve reliability of the Cochrane’s risk of bias tool, help authors in making decisions about risk of bias and help in making reliable decisions in healthcare.
Highlights
An important part of the systematic review methodology is appraisal of the risk of bias in included studies
In the main analysis we reported only analysis of attrition bias for included Cochrane reviews with a single judgment (i.e. Cochrane reviews with only one domain for attrition bias, and one judgment in that one domain), regardless of the number of supporting explanations that were provided for that judgment
Among 955 Cochrane systematic reviews published from July 2015 to June 2016 we included 729 Cochrane reviews in the main analysis
Summary
An important part of the systematic review methodology is appraisal of the risk of bias in included studies. Cochrane systematic reviews are considered golden standard regarding systematic review methodology, but Cochrane’s instructions for assessing risk of attrition bias are vague, which may lead to inconsistencies in authors’ assessments. The aim of this study was to analyze consistency of judgments and support for judgments of attrition bias in Cochrane reviews of interventions published in the Cochrane Database of Systematic Reviews (CDSR). An important part of the systematic review methodology is appraisal of the risk of bias (RoB) in included studies. In Cochrane systematic reviews RoB is appraised using Cochrane RoB tool, which has seven domains; one of them is called ‘incomplete outcome data (attrition bias)’. Incomplete outcome data can yield attrition bias due to amount, nature or handling of incomplete outcome
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