VALIDATION DATA

The exploratory factor analysis (EFA) of the 'now' function data and 'now' and 'at diagnosis' emotional and psychological well-being data showed that these scales were unidimensional and had high internal consistency (Cronbach's alpha >0.9). However, EFA of the 'at diagnosis' function data resulted in two factors, and there was no clinically meaningful distinction between the groups of items loading onto each factor. Therefore, Rasch analysis was employed to aid item reduction and assess unidimensionality more rigorously.

The validation study included a sample size of more than 50 for each domain, and the intraclass correlation coefficient (ICCagreement) was greater than 0.8 in each domain, indicating good reliability. The standard error of measurement (SEMagreement) for each domain ranged from 9.3 to 11.9 on a scale out of 100.

The candidate items for the PMR-IS were derived from a conceptual framework and a list of symptoms and effects of PMR identified through literature synthesis and qualitative research. The PMR questionnaire was completed by 28 participants with PMR, along with the QQ-10 questionnaire used to assess the face validity, feasibility, and utility of patient healthcare questionnaires. The results showed a high mean value score of 79% (SD 12) and a low burden score of 21% (SD 18), indicating good face validity and acceptability to patients.

The Rasch analysis used a partial credit model and iteratively deleted the least well-fitting items until satisfactory fit statistics were achieved for unidimensional scales. At the end of the process, a 9-item functional scale and a 4-item psychological and emotional well-being scale were created. The only item showing differential item functioning (DIF) was 'take your shoes or socks on or off', which showed DIF for gender in the 'now' dataset.