Standard pairwise meta-analysis of four AB trials (comparing treatment B relative to A). (a) No differences in the effect modifier ‘disease severity’ across studies and therefore no between-study heterogeneity. (b) Extreme differences in the effect modifier across studies and therefore between-study heterogeneity. (Not useful to pool the studies). (c) Differences in the distribution of the effect modifier across studies and therefore between-study heterogeneity. Given the inclusion of both severe and moderate patients in each of the studies there is also within-study heterogeneity, but this cannot be observed without access to subgroup or individual patient-level data.
Jansen and Naci BMC Medicine 2013 11:159 doi:10.1186/1741-7015-11-159