Significance testing in mutagen screening: the dependence of statistical power on the control sample size

The continuous accumulation of control data in multicellular mutagen screening systems prompted us to study the dependence of the statistical power on the size of the control sample (for fixed control values). Two widely used screening systems were chosen: dicentric chromosomes in human lymphocytes and recessive sex-linked lethals in Drosophila melanogaster. The power increases rapidly at first as the control sample size increases, then levels off at a few tens of thousands of control units tested and thereafter remains almost constant up to the historical control. The practical implications from our study are discussed.