Look out for results that are reported for a selected subgroup within a study or systematic review.
Comparisons of interventions often report results for selected groups of subjects, called “subgroups”. This is because people want to know whether the effect of an intervention varies for different types of subject (e.g. in a different soil type).
But subgroup 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 in the effect of an intervention between different subgroups.
REMEMBER: Intervention effects that are based on the results for subgroups within intervention comparisons may be misleading.