Comparative characteristics of estimators of the mean responding cell density in limiting dilution assays

The statistical problems associated with estimating the mean responding cell density in the limiting dilution assay (LDA) have largely been ignored. We evaluate techniques for analyzing LDA data from multiple biological samples, assumed to follow either a normal or gamma distribution. Simulated data is used to evaluate the performance of an unweighted mean, a log transform, and a weighted mean procedure described by Taswell (1987). In general, an unweighted mean with jackknife estimates will produce satisfactory results. In some cases, a log transform is more appropriate. Taswell's weighted mean algorithm is unable to estimate an accurate variance. We also show that methods which pool samples, or LDA data, are invalid. In addition, we show that optimization of the variance in multiple sample LDA's is dependent on the estimator, the between-organism variance, the replicate well size, and the numberof biological samples. However, this optimization is generally achieved by maximizing biological samples at the expense of well replicates.