Sample size determinations using examples drawn from the NCTR Collaborative Behavioral Teratology Study data.

Abstract

The Collaborative Behavioral Teratology Study (CBTS) introduced the coefficient of detection (CD) as an estimate of the minimum difference needed to detect a significant difference among groups. The CD was used to provide an index of the sensitivity of the tests used. As originally described, the CD is not suitable for estimates of two or more group minimally significant differences because it is based on a one-group formula. Also, the CD operates at a power of 50% in the one-group case (see preceding paper). These drawbacks leave questions about the sensitivity of the tests used in the CBTS unanswered. Using examples drawn from the CBTS data base, a more standard method of approaching this question has been taken here based on power calculations. Using the means and standard deviations from the vehicle controls, alpha = 0.05, 1-beta = 0.80, and 20, 30 or 40% group mean difference sizes, required samples sizes were determined for all the behavioral measures reported in the CBTS final report for Experiment 1. The results showed that most of the measures can detect 30% group mean differences in a two-group, two-tailed t-test situation with group sizes of less than or equal to 20 litters per group. More complex calculations are required when more than two groups are planned. Multigroup designs require additional assumption about the distribution of group differences and accordingly are more difficult to specify.(ABSTRACT TRUNCATED AT 250 WORDS)