Statistical considerations sample size power multiple comparisons

1. Phenotypic studies

Phenotypic studies are generally small to moderate in size and often consist of a highly selected sample. The differences reported between cases and controls are difficult to evaluate. A large issue is that of separating the results from cases who have cancer and the effects that cancer may exert on the phenotype from a susceptibility to develop cancer. As phenotyping to date is labor intensive, this issue is unlikely to be resolved soon. The cohort studies represented by Bonassi et al.14 give the best insight into the likely true nature of the effect of lower DNA repair capacity on cancer, an approximately twofold increase for those with a higher level of aberrations. In phenotypic studies the statistical power is much stronger than genotyping studies because continuous variables can be used. Multiple comparisons can become a problem whenever the group is split into subgroups, as in any study.

2. Genotyping studies

The sample size is always a problem in this type of investigations. However, relatively common genotypes were considered, so that statistical power is usually moderate. An important methodological problem refers to multiple comparisons (Table 7.5): Splitting the data into several subgroups increases the probability of finding statistically significant results. For this reason, the reported interactions in Table 7.5 have to be considered cautiously.

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