Smallest detectable change and test‐retest reliability of a self‐reported outcome measure: Results of the Center for Epidemiologic Studies Depression Scale, General Self‐Efficacy Scale, and 12‐item General Health Questionnaire

Rationale, aims, and objectives This study aims to examine the smallest detectable change (SDC) and test‐retest reliability of the Center for Epidemiologic Studies Depression Scale (CES‐D), General Self‐Efficacy Scale (GSES), and 12‐item General Health Questionnaire (GHQ‐12). Method We tested 154 young adults at baseline and 2 weeks later. We calculated the intra‐class correlation coefficients (ICCs) for test‐retest reliability with a two‐way random effects model for agreement. We then calculated the standard error of measurement (SEM) for agreement using the ICC formula. The SEM for agreement was used to calculate SDC values at the individual level (SDC ind ) and group level (SDC group ). Results The study participants included 137 young adults. The ICCs for all self‐reported outcome measurement scales exceeded 0.70. The SEM of CES‐D was 3.64, leading to an SDC ind of 10.10 points and SDC group of 0.86 points. The SEM of GSES was 1.56, leading to an SDC ind of 4.33 points and SDC group of 0.37 points. The SEM of GHQ‐12 with bimodal scoring was 1.47, leading to an SDC ind of 4.06 points and SDC group of 0.35 points. The SEM of GHQ‐12 with Likert scoring was 2.44, leading to an SDC ind of 6.76 points and SDC group of 0.58 points. Conclusion To confirm that the change was not a result of measurement error, a score of self‐reported outcome measurement scales would need to change by an amount greater than these SDC values. This has important implications for clinicians and epidemiologists when assessing outcomes.