Look out for results that are reported for selected subgroups within a study or systematic review. Subgroup analyses may be misleading.
Average effects do not apply to everything. However, comparisons of interventions often report results for selected groups of subjects in an effort to assess whether the effect of an intervention is different for different types of subjects (e.g. different species or sites in different geographical locations). These analyses are often poorly planned and reported. Most differential effects suggested by these “subgroup results” are likely to be due to the play of chance and are unlikely to reflect true differences.
REMEMBER: Findings based on results for subgroups of subjects within an intervention comparison may be misleading.