Background Employment studies in arthritis have emphasised the importance of decreased productivity at work, or presenteeism. Yet, how to best measure presenteeism remains challenging. The “Work Limitations Questionnaire” (WLQ) is frequently used. A drawback is that it measures the amount of time people are limited, but not the degree to which they are limited. In contrast, the “Workplace Activity Limitations Scale” (WALS) measures the degree of limitation, but not the time. We modified the response keys to the WALS and WLQ to measure both degree of difficulty and amount of time with difficulty.
Objectives Our objective was to evaluate measurement properties, i.e. internal consistency and construct validity, of the WALS and WLQ with combined scores from dual answer keys.
Methods A cross-sectional study used baseline data from the RCT of an employment intervention, the “Making It Work” Program. Participants were recruited from BC, Alberta and Ontario. Inclusion criteria included: having inflammatory arthritis, currently employed, age 19–59, and having concerns about arthritis affecting ability to work. 364 participants were included (RA:195, PsA:54, SLE:46, AS:69; 77% female, mean (SD) age: 45.9 (9.8) yrs). Combined scores were obtained by i) multiplying, and ii) adding, the scores of difficulty and time answer keys at the item level. No significant differences were observed between the additive and multiplicative models. Hence, we report on the multiplicative model, which reflects consumers' preference. Internal consistency was analyzed using Cronbach's alphas; construct validity by measuring correlation (Spearman coefficients) between WALS or WLQ subscales and constructs such as work productivity activity impairment (WPAI), risk of impending work loss (work instability, RA-WIS), disease measures, and job characteristics.
Results Analyses at the item level revealed a strong floor effect (WALS: 16% to 56%; WLQ: 27% to 81% of answers for all items except one) but no ceiling effect, likely reflecting the relatively low limitation expected in a working sample. High (i.e. ≥0.7) internal consistency (α 0.70–0.82) was found for WALS and all WLQ subscales except WLQ Physical Demands (0.67). As a priori hypothesized, moderate correlation was observed between the time (0.33–0.57), or combined (0.44–0.61), scores of WLQ subscales and WPAI, or WIS; and correlation was consistently higher for combined than time scores. Moderate correlation was also observed between degree of difficulty (0.70–0.77), or combined (0.60–0.69), scores of the WALS and WPAI, or WIS. Contrary to our hypothesis, the WALS combined score did not have a higher correlation with WPAI, or WIS, than degree of difficulty score. Correlations with disease (fatigue, pain, physical function, depression) and job characteristics (job demand, autonomy, social support at work, commuting difficulty) with time, difficulty, or combined scores of the WALS and WLQ met the a priori hypothesized correlation levels.
Conclusions Our previous research confirmed the value of measuring both degree of difficulty and amount of time, showing they measure different concepts, which can serve different purposes and are both important to patients. This research demonstrates that applying a dual answer key to two validated instruments shows good initial indicators of internal consistency and construct validity.
Disclosure of Interest None declared