The power of a cohort study depends on several factors: (1) the number of subjects enrolled in the cohort, (2) the time for which each subject is followed-up, (3) the rate at which events (PRCa) occur in the cohort, (4) the frequency of ‘exposure’ to the hypothesised risk factor in the cohort (in this case, the frequency of the CYP3A4 genotypes or/and VDRTaqI genotypes, which had been hypothesised to be associated with increased risk of developing PRCa), (5) the size of RR which the investigator want to detect. The gene discussed is CYP3A4; the disease is pure red-cell aplasia.