Circadian patterns of rat anterior pituitary and target gland hormones in serum: determination of the appropriate sample size by statistical power analysis

(1) Experiments were performed to ascertain circadian fluctuations in the serum concentrations of luteinizing hormone (LH), follicle stimulating hormone (FSH), prolactin (PRL), testosterone (T), growth hormone (GH), thyroid stimulating hormone (TSH), and corticosterone in rats. (2) Serum LH, TSH, T and corticosterone varied throughout the 24-hr period, but the magnitude of change for LH, TSH and T was small compared with the range of individual animals' hormone concentrations at a given time point. (3) Statistical power analyses were performed on the LH, TSH, and corticosterone data in order to determine retrospectively the sample size necessary to reduce the possibility of a Type II (beta) statistical error to an acceptable level. (4) The results of these analyses indicate that many similar studies already published used too few animals and thus were biased toward not rejecting the null hypothesis.