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

Objectives: 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.