- Separating patients into subgroups and then performing analysis within the subgroup is how observational studies are performed.
- RCTs, however, run into trouble with subgroup analysis.
Thus, even though a well-designed clinical trial may have validity, its subgroup analysis may not.
- There should not be too many subgroups.
- Subgroups should be identifiable by features apparent before randomisation.
- There should be some test of interaction, rather than within-subgroup analysis.
- each subgroup may be underpowered to detect the desired effect size
- If enough subgroup analyses are performed, eventually one will be positive by chance alone.
How do I make my subgroup analysis appear more credible?
- Use within-study comparison rather than within-subgroup comparison
- Specify the subgroups in the study design, rather than post-hoc
- It helps if the effect size is large and if there is compelling biological rationale.