A quantity, λ2 is defined as the discrepancy between an actual and an assumed statistical model. An estimator is developed for the case in which data are grouped. The estimator is based on Pearson's X2 goodness-of-fit statistic, and permits comparisons of lack of fit among data sets having dissimilar sample sizes (N's) or numbers of bins (n's). Although is equivalent to X2/N when N is large and n is fixed, it is less biased and has smaller variance than X2/N. These effects are most pronounced in small samples and in sparse-data situations. Use and interpretation of as a data-analytic tool are discussed, and an example involving several sets of data from particle-scattering experiments is presented.