Psychometric evaluation and community norms of the Somatic Symptom Scale-8 based on a representative German sample

The Somatic Symptom Scale-8 (SSS-8) is a brief self-report instrument assessing somatic symptom burden derived from the PHQ-15. The aims of the present study were providing updated population norms for Germany based on a new representative sample and enabling a direct comparison with the original 2012 normative sample. In addition, the study aimed to examine the psychometric properties of the SSS-8, evaluate construct validity, and test factorial validity and measurement invariance.

A representative German community sample was surveyed in 2021 (N = 2,515; 51.6% female; age range: 16-101 years). Construct validity was evaluated through latent correlations with measures of depression (PHQ-2), anxiety (GAD-2), well-being (WHO-5), and loneliness (UCLA-3). Health care utilization was predicted using negative binomial regression. Factorial validity was examined using confirmatory factor analysis with WLSMV estimation (unidimensional model and second-order factor model). Measurement invariance was tested across gender, age, gender × age, depression (PHQ-2 ≥ 3), and anxiety (GAD-2 ≥ 3). Population norms were established using SCAM-smoothed cumulative percentiles.

Internal consistency was high (ω = .922, 95% CI [.915,.929]). The higher-order model (CFI = .979, TLI = .966, SRMR = .046; RMSEA = .091, 90% CI [.083,.099]) showed better fit than the unidimensional model (CFI = .961, TLI = .945, SRMR = .054; RMSEA = .115, 90% CI [.108,.123]). Strict invariance was supported for depression and anxiety. For gender, age, and gender × age, partial scalar invariance was established after freeing one item intercept (headaches). Latent correlations supported convergent and discriminant validity with depression (r = .81), anxiety (r = .76), well-being (r = -.67), and loneliness (r = .57). Each one-point increase in SSS-8 scores was associated with a 9.7% increase in doctor visits (IRR = 1.097, 95% CI [1.082, 1.112]). A cross-cohort comparison with the original normative sample (2012; N = 2,484) revealed a significant increase in somatic symptom burden (d = -0.40, 95% CI [-0.45, -0.34]).

The SSS-8 demonstrates good psychometric properties in a representative German community sample. Updated population norms are provided for clinical use. The observed increase in somatic symptom burden underscores the necessity of updating population norms at regular intervals.