IMPACT OF STRATIFICATION IMBALANCE ON PROBABILITY OF TYPE-I ERROR

The impact of ignoring the stratification effect on the probability of a Type I error is investigated. The evaluation is in a clinical setti ng where the treatments may have different response rates among the st rata. Deviation from the nominal probability of a Type I error, alpha, depends on the stratification imbalance and the heterogeneity in the response rates; it appears that the latter has a larger impact. The pr obability of a Type I error is depicted for cases in which the heterog eneity in the response rate is present but there is no stratification imbalance. Three-dimensional graphs are used to demonstrate the simult aneous impact of heterogeneity in response rates and of stratification imbalance.