Reliability and validity of the Norwegian version of the Functional Outcomes of Sleep Questionnaire

The aim of this study was to validate the Norwegian version of a self-administered 30-item quality of life questionnaire designed to assess disorders of excessive sleepiness, the Functional Outcomes of Sleep Questionnaire (FOSQ). In total 226 patients previously evaluated for obstructive sleep apnea were included in the study. The patients received a postal questionnaire with the FOSQ, the Short Form 36 (SF-36) questionnaire, and a scale for assessment of excessive daytime sleepiness, the Epworth sleepiness scale (ESS). Among the 178 respondents, all five subscales of the FOSQ showed good internal consistency reliability (Cronbach's alpha = 0.84-0.93). Test-retest on average 18 days apart was satisfactory with intraclass correlation coefficients ranging from 0.61 to 0.86. The pattern of Spearman's rank correlation coefficients between FOSQ scales and related and unrelated scales SF-36 scales gave support to the construct validity of the FOSQ. In conclusion, the Norwegian translation of the FOSQ showed satisfactory internal consistency reliability, test-retest reliability and construct validity, in line with the original version.