"How fatigued do you currently feel?" Convergent and discriminant validity of a single-item fatigue measure.

The main aim of this study was to establish the convergent and discriminant validity of a single-item measure of daily fatigue ("How fatigued do you currently feel?") in a daily diary context. Convergent validity of our measure was examined by relating it to a validated multiple-item measure of fatigue (Profile of Mood States; McNair, Lorr, & Droppelman, 1971) and to other daily (work-home interference, sleep complaints, work-related effort) and global (fatigue, health complaints, work-home interference, job pressure) measures that are conceptually related to fatigue. Discriminant validity was assessed by relating the single-item fatigue measure to daily (work pleasure) and global (job control, social support, motivation to learn) measures that are conceptually distinct from fatigue. Data were collected among 120 academic staff members, who completed a general questionnaire (tapping the global measures under study) and who took part in a 9-d daily diary study (3 measurements daily). Correlation patterns and multilevel analyses revealed strong and significant associations between the single-item fatigue measure and the variables incorporated to assess convergent validity (especially with the POMS: r=0.80), thus supporting the convergent validity of our measure. Relations with variables included to examine discriminant validity were weak or insignificant, supporting the discriminant validity of the single-item fatigue measure. Despite this study's limitations (i.e., exclusive use of self-reporting, specific sample) we conclude that this single-item fatigue measure offers a valid way to assess daily fatigue.