VALIDATION DATA

Internal consistency was confirmed as acceptable with strong Cronbach’s alpha for the total score and all domain scores (ɑ = 0.94-0.99).

Test-retest reliability was assessed by evaluating the reproducibility of MDF-Fibro-17 scores over the time period between Baseline and Day 8 and from Week 5 to Week 6. All ICCs (Spearman Brown) exceeded the required 0.70 level, for baseline versus day 8, ICCs ranged from 0.71 to 0.82 (median of 0.74), and for Week 5 versus Week 6 all exceeded 0.90.

The structure of the MDF-Fibro-17 is assumed to be an accurate representation of the key concepts in FM-related fatigue. This assumption is based upon the categorization of concepts developed through a review of the grounded theory analysis of the qualitative work, with additional support from the cognitive debriefing work, which indicated that the items within the measure were relevant and understandable. The item level data and initial confirmatory factor analysis, conducted using data from the previous cross-sectional validation study, also supported the content validity of the proposed MDF-Fibro-17.

The Confirmatory Factor Analysis supported the 17-item, 5 domain structure of this instrument as the best fit of the data: comparative fit index (CFI) and non-normed fit index (NNFI) were 0.997 and 0.992 respectively, standardized root mean square residual (SRMR) was 0.010 and the root mean square error of approximation (RMSEA) was 0.06.