Subgroup analysis in randomized controlled trials appeared to be dependent on whether relative or absolute effect...

To assess whether relative or absolute effect measures were used in subgroup analyses of randomized controlled trials (RCTs) and study whether conclusions would change if subgroup effects were calculated on a different scale than reported.

We studied all 327 RCTs published in 2010 in five major medical journals. For trials with a dichotomous primary outcome, we extracted reported main and subgroup effect measures. If crude subgrouping data were reported, we calculated the subgroup effects on both relative and absolute scales.

Of the 229 RCTs with a dichotomous primary outcome, 120 (52%) performed subgroup analyses. In 106 of these 120 (88%) RCTs, relative effect measures were used for subgroup analyses, whereas an absolute scale was used in 9 (8%) trials. Two (2%) RCTs reported both relative and absolute subgroup effects. Crude data of the subgroups could be extracted in 41 of the 120 (34%) RCTs. Calculating subgroup effects on a different scale than reported lead to a change in conclusion in 17% of the 41 trials.

Almost all RCTs used relative effect measures for subgroup analyses. Interpretation of subgroup effects, however, appeared to be dependent on whether relative or absolute effect measures were used.