Experimental design and statistical analysis considerations for in vitro mammalian cell transformation assays with BALB/3T3 cells

Many mammalian cell assays testing for mutagenic activity have common features which cause statistical estimation and analysis problems. Such assays measure the number of cell alterations occurring in a plate containing an unknown number of cells at risk. The number of cells at risk can be estimated from a parallel cytotoxicity study. While the Poisson distribution has been assumed to apply to standardized frequencies, this is questioned. The failure of standardized frequencies to follow a Poisson distribution is attributed to the relatively small and dosage-dependent number of susceptible cells per plate. A minimum number of such cells per plate or random cluster of plates has been determined for each dose so that the measured variable approximates a Poisson distribution. A transformation is suggested to achieve reasonable normality and variance equality, thereby allowing the use of parametric analysis of variance and regression methods and an estimation of required sample sizes.