Correlation coefficients between biological measurements and clinical
scales are often calculated in psychiatric research. Calculating numer
ous correlations increases the risk of a type I error, i.e., to errone
ously conclude the presence of a significant correlation. To avoid thi
s, the level of statistical significance of correlation coefficients s
hould be adjusted. Threshold levels of significance for correlation co
efficients were adjusted for multiple comparisons in a set of k correl
ation coefficients (k = 1, 5, 10, 20, 50, 100) by Bonferroni's correct
ion. Significant correlation coefficients were then calculated accordi
ng to sample size. The change in the threshold values of significance
is larger when the number of correlations goes from 1 to 5 than when i
t goes from 50 to 100. A correlation coefficient, statistically signif
icant at 5% when calculated alone, can be under the threshold level of
significance when calculated even among a few other coefficients. Foc
using on the most relevant variables or the use of multivariate statis
tics is advocated. Biol Psychiatry 1998;44:775-777 (C) 1998 Society of
Biological Psychiatry.