A statistical examination of historical controls for mouse bone marrow cytogenetic assays

Data from 1,111 controls from assays run over 11 years are examined to determine a most powerful statistical procedure for detecting a mutagenic effect. It is concluded that the data do not show a constant probability of chromosomal abnormalities and that the data do not fit a simple Poisson or binomial distribution. Empirical Bayes techniques are used to derive a test that declares an effect if three or more cells in a group of 50 tested contain abnormalities.