Measurement invariance of the Illness Invalidation Inventory (3*I) across language, rheumatic disease and gender

The Illness Invalidation Inventory (3*I) assesses patients' perception of responses of others that are perceived as denying, lecturing, not supporting and not acknowledging the condition of the patient. It includes two factors: 'discounting' and 'lack of understanding'. In order to use the 3*I to compare and pool scores across groups and countries, the questionnaire must have measurement invariance; that is, it should measure identical concepts with the same factor structure across groups. The aim of this study was to examine measurement invariance of the 3*I across rheumatic diseases, gender and languages.

Participants with rheumatic disease from various countries completed an online study using the 3*I, which was presented in Dutch, English, French, German, Portuguese and Spanish; 6057 people with rheumatic diseases participated. Single and multiple group confirmatory factor analyses were used to test the factorial structure and measurement invariance of the 3*I with Mplus.

The model with strong measurement invariance, that is, equal factor loadings and thresholds (distribution cut-points) across gender and rheumatic disease (fibromyalgia vs other rheumatic diseases) had the best fit estimates for the Dutch version, and good fit estimates across the six language versions.

The 3*I showed measurement invariance across gender, rheumatic disease and language. Therefore, it is appropriate to compare and pool scores of the 3*I across groups. Future research may use the questionnaire to examine antecedents and consequences of invalidation as well as the effect of treatments targeting invalidation.