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Table 4 Reasons for non-conduct of a meta-analysis for the systematic review’s main outcome even when ≥2 studies were included in the systematic review (N = 51 of 125 reviews that included ≥2 studies)

From: Outcome choice and definition in systematic reviews leads to few eligible studies included in meta-analyses: a case study

ReasonNumber of systematic reviews (N = 51)
n (%)
When meta-analyzable data1 for the review’s main outcome domain were NOT REPORTED by ≥2 studies (n = 23 reviews)
  < 2 studies measured the review’s main outcome21(41)
  < 2 studies reported any data for the review’s main outcome1(2)
  < 2 studies reported any meta-analyzable data1for the review’s main outcome1(2)
When meta-analyzable data1 for the review’s main outcome domain were REPORTED by ≥2 studies (n = 28 reviews)2
Reasons related to inconsistencies in outcome elements
 Studies used inconsistent specific measurements16(31)
 Studies used inconsistent specific metrics0(0)
 Studies used inconsistent methods of aggregation0(0)
 Studies reported data at inconsistent time-points9(18)
Reasons related to heterogeneity
 Studies were clinically heterogeneous7(14)
 Studies were methodologically heterogeneous2(4)
 Studies were statistically heterogeneous0(0)
  1. 1For categorical outcomes, we considered data to be meta-analyzable if either of the following scenarios were met [1]: total number of participants and number of participants with the outcome of interest were reported for each study arm; and [2] the between-group treatment effect (e.g., relative risk, odds ratio) and an estimate of uncertainty (e.g., 95% confidence interval) were reported. For continuous and time-to-event outcomes, we considered data to be meta-analyzable if either of the following scenarios were met [1]: mean and estimate of uncertainty (e.g., standard deviation) were reported for each study arm; and [2] the between-group treatment effect (e.g., mean difference) and an estimate of uncertainty (e.g., 95% confidence interval) were reported
  2. 2More than one reason could apply