The large ISIS-2 trial1 involved 17,000 patients. The beneficial effect of aspirin for patients having a heart attack was very substantial and equal to the effect of streptokinase (a powerful clot dissolving medication). Both were life saving medications. (The trial result for aspirin was very statistically significant (2p <.00001) with much less than a 1/1000 chance of these findings being the result of chance.)
The ISIS-2 investigators note: "When in a trial with a clearly positive overall result, many subgroup analyses are considered, false negative results in some particular subgroups must be expected."
The ISIS-2 authors then give as an example that “subdivision of the patients in ISIS-2 with respect to their astrological birth sign appears to indicate that for persons born under Gemini or Libra, there was a slightly adverse effect of aspirin on mortality (9% increase, SD 13; NS), while for patients born under all other astrological signs there was a striking beneficial effect (28% reduction, SD 5; 2p <0.00001).”
The subgroup of analysis suggesting that Gemini and Libra had an adverse effect rather than a beneficial effect with aspirin was not a true relation. These patients would benefit from aspirin to an equal degree as the rest of the group.
Subgroup analysis can lead to findings that are incorrect.
In particular, if a given therapy has a highly significant and strongly beneficial effect for the group as a whole, a subgroup analysis that results in the unexpected finding that certain subgroups do not have benefit, is frequently incorrect.
In fact, it is more likely that the unexpected subgroup finding which runs counter to the group finding, is simply not valid.
1. ISIS-2 (Second International Study of Infarct Survival). Lancet 1988: ii: 349-360 (pages of interest 356-357)