Reliability and validity of a Norwegian version of the Epworth sleepiness scale

The objective of this study was to assess the reliability and validity of a Norwegian version of the self-administered Epworth sleepiness scale (ESS). Two samples responded to the ESS: (1) 226 patients previously evaluated for obstructive sleep apnea, of whom 51 also responded to a retest 2 weeks later, and (2) 37 ambulant patients complaining of excessive daytime sleepiness, who were referred to multiple sleep latency testing (MSLT). We assessed internal consistency reliability with Cronbach's alpha and test-retest reliability with weighted kappa (Kw) or an intraclass correlation coefficient (ICC). The validity of the Norwegian ESS was assessed by correlating ESS item and total scores with the number of times a patient fell asleep and the mean latency found on the MSLT. Internal consistency reliability, as assessed with Cronbach's alpha, was 0.84 (n = 154). Test-retest reliability for the eight ESS items ranged from Kw of 0.61 to 0.80 (n = 50) and for the total score. ICC was 0.81.There was only fair to moderate correlation of ESS item and total scores with MSLT variables, mainly in a subset of patients with total ESS score > 10. The Norwegian version of the ESS had acceptable internal consistency and test-retest reliability. The association of the ESS items and total score with the MSLT was only fair to moderate, in line with previous studies.