The ABC breast cancer cluster: the bad news about a good outcome

To the Editor: An editorial by Stewart alludes to the problem of silent multiple comparisons when interpreting P values from cancer cluster investigations.1 Visible multiplicities such as occur with pre-specified subgroup analyses or sequential monitoring of trials are difficult enough, but at least in these circumstances we know how many multiple comparisons are under consideration. More difficult are silent multiplicities such as occur with cluster investigations (and also with publication bias2 or reporting bias3) where we do not know how many multiple comparisons should be considered.