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Psychometric properties and population norms of the Brief Symptom Inventory-18 (BSI-18) based on a representative German sample
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DOI：10.3389/fpsyt.2026.1877721.png)
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Abstract
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BackgroundThe Brief Symptom Inventory-18 (BSI-18) is a widely used self-report measure of psychological distress assessing somatization; depression; and anxiety. Although German population norms were published in 2017 based on data from 2009; updated psychometric evidence using modern statistical methods is lacking.MethodsThe BSI-18 was administered to a random probability sample representative of the German residential population aged 16 years and older (N = 2; 519; 52.5% women; age range 16–96 years); collected between December 2020 and March 2021. Psychometric evaluation included confirmatory factor analysis (CFA) with the weighted least squares mean and variance adjusted (WLSMV) estimator; internal consistency (McDonald’s omega and Cronbach’s alpha with bootstrap confidence intervals); measurement invariance across gender; age; and clinical screening status; construct validity via latent correlations with the PHQ-9; GAD-7; and CFQ-11; and age- and gender-stratified population norms. Cross-cohort comparisons were conducted with the normative sample from 2009.ResultsThe correlated three-factor model showed good fit (χ2(132) = 644; CFI = 0.986; RMSEA = 0.039; 90% CI [0.036; 0.042]). Internal consistency was good to very good (Global Severity Index [GSI]: ω = 0.966; subscales: ω = 0.855 to 0.893). Full measurement invariance was established across gender; age; and clinical screening status. Latent correlations confirmed the hypothesized convergent validity pattern for depression (PHQ-9: r = 0.912); anxiety (GAD-7: r = 0.889) and somatization (CFQ-11: r = 0.580). Women scored higher on all scales (d = -0.11 to -0.14). Cross-cohort comparisons revealed no significant change in the GSI (d = -0.05; p = 0.105) or depression (d = 0.04; p = 0.200); whereas somatization (d = -0.11; p < 0.001) and anxiety (d = -0.07; p = 0.006) scores were significantly lower in 2020/21.ConclusionsThe German BSI-18 demonstrated sound psychometric properties; including full measurement invariance. The updated population norms provide a useful reference for clinical practice. Cross-cohort comparisons revealed no general increase in distress during the COVID-19 pandemic.
Keywords:
depression
psychometric properties
anxiety
psychological distress
population norms
measurement invariance
somatization
BSI-18
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