Validity

Validity refers to whether a measurement tool can accurately measure what the researcher intends to measure. Two general forms of validity, construct validity and content validity, were assessed for the K-FJS. Construct validity is the degree to which an instrument measures the trait or theoretical construct that it is intended to measure. To assess the construct validity, it was hypothesized that the K-FJS score would have a moderate to strong positive correlation with other PROM scoring systems (i.e., WOMAC and SF-36). Pearson's correlations coefficient was also calculated. Values greater than 0.6 indicated a strong correlation.¹⁶⁾ Content validity expresses how well the questionnaire covers all symptoms experienced by patients. The content validity was assessed for floor and ceiling effects. Floor effects include the proportion of patients scoring the lowest possible, whereas a ceiling effect expresses patients scoring the highest possible. Floor and ceiling effects of less than 15% of patients were considered to be adequate.¹⁷⁾