Comparison of human hair cortisol concentration stability for 1‐year and 2‐year test–retest intervals

Human hair cortisol concentration (HCC) has previously been found to be highly stable for a 1-year interval (r = 0.73) in terms of a product-moment correlation. The present study aimed to replicate this finding and compare HCC stability regarding 1-year and 2-year test-retest intervals. Female university students (N = 39) provided hair strands twice (t1 and t2) at intervals of 1 (n = 21) or 2 years (n = 18). Multiple regression analysis predicting HCC at t2 revealed a significant interaction term (HCC at t1 × time interval condition). It was determined that HCCs were substantially related for the 1-year interval but unrelated for the 2-year interval. The findings were not attributable to potential influences, such as hair treatment. The product-moment correlation showed nearly identical consistency with previous research regarding the 1-year test-retest interval. There was no significant product-moment correlation for the 2-year interval. Overall, these findings indicate that within a temporal framework of 1 year, HCCs may be stable predictors in correlational studies where the focus is on the rank orders of measured values.