Establishing construct validity for the thyroid-specific patient reported outcome measure (ThyPRO): an initial examination

Objective  To establish a reliable and valid scale structure of a patient-reported outcome measuring thyroid-specific quality of life. Methods  The 98-item ThyPRO questionnaire was administered to patients with benign thyroid diseases at two university hospitals. Multi-trait scaling was performed, evaluating lack of convergent validity (item-own scale polyserial correlation <0.40) or lack of discriminant validity (item-other scale correlation higher than item-own scale correlation) of the hypothesized scale structure. Analyses were repeated in clinical and sociodemographic subgroups and with Pearson correlations. Reliability was estimated by Cronbach’s α, both conventionally and with polychoric correlations. Results  In total, 904 patients (69%) responded. Initial multitrait scaling analysis identified 25 scaling errors. Twelve items were omitted from the scale structure, and a re-analysis showed complete convergent validity and only two instances of lack of discriminant validity. Pearson correlations yielded similar results. Across all subgroups, convergent validity was complete, and discriminant validity was found in 99.2% of tests. Lack of discriminant validity was mainly between physical symptoms and psychological and disease-impact scales. Cronbach’s α was acceptable (>0.70, >0.80 with polychoric correlations) for all 13 scales. Conclusion  A reliable scale structure displaying complete convergent and almost complete discriminant validity was established in general analyses and in distinct clinical subgroups of patients with benign thyroid diseases. Researchers who want to use the ThyPRO may contact the first author (T.Watt@rh.dk).