BACKGROUND: Four dimensions of inner strength were previously identified in a meta-theoretical analysis: firmness, creativity, connectedness, and flexibility.

OBJECTIVE: The aim of this study was to develop an Inner Strength Scale (ISS) based on those four dimensions and to evaluate its psychometric properties.

METHOD: An initial version of ISS was distributed for validation purpose with the Rosenberg Self-Esteem Scale, the resilience scale, and the sense of Coherence Scale. A convenience sample of 391 adults, aged 19-90 years participated. Principal component analysis (PCA) and confirmatory factor analysis (CFA) were used in the process of exploring, evaluating, and reducing the 63-item ISS to the 20-item ISS. Cronbach's alpha and test-retest were used to measure reliability.

RESULTS: CFA showed satisfactory goodness-of-fit for the 20-item ISS. The analysis supported a fourfactor solution explaining 51% of the variance. Cronbach's alpha on the 20-item ISS was 0.86, and the test-retest showed stability over time (r=0.79).

CONCLUSION: The ISS was found to be a valid and reliable instrument for capturing a multifaceted understanding of inner strength. Further tests of psychometric properties of the ISS will be performed in forthcoming studies.