THE VALIDITY AND REPRODUCIBILITY OF A WORK PRODUCTIVITY AND ACTIVITY IMPAIRMENT INSTRUMENT

The construct validity of a quantitative work productivity and activit y impairment (WPAI) measure of health outcomes was tested for use in c linical trials, along with its reproducibility when administered by 2 different methods. 106 employed individuals affected by a health probl em were randomised to receive either 2 self-administered questionnaire s (self administration) or one self-administered questionnaire followe d by a telephone interview (interviewer administration). Construct val idity of the WPAI measures of time missed from work, impairment of wor k and regular activities due to overall health and symptoms, were asse ssed relative to measures of general health perceptions, role (physica l), role (emotional), pain, symptom severity and global measures of wo rk and interference with regular activity. Multivariate linear regress ion models were used to explain the variance in work productivity and regular activity by validation measures. Data generated by interviewer -administration of the WPAI had higher construct validity and fewer om issions than that obtained by self-administration of the instrument. A ll measures of work productivity and activity impairment were positive ly correlated with measures which had proven construct validity. These validation measures explained 54 to 64% of variance (p < 0.0001) in p roductivity and activity impairment variables of the WPAI. Overall wor k productivity (health and symptom) was significantly related to gener al health perceptions and the global measures of interference with reg ular activity. The self-administered questionnaire had adequate reprod ucibility but less construct validity than interviewer administration. Both administration methods of the WPAI warrant further evaluation as a measure of morbidity.