Article Text

Download PDFPDF
Original article
Minimum important difference of the Epworth Sleepiness Scale in obstructive sleep apnoea: estimation from three randomised controlled trials
  1. Sarah Crook1,
  2. Noriane A Sievi2,
  3. Konrad E Bloch2,3,
  4. John R Stradling4,
  5. Anja Frei1,
  6. Milo A Puhan1,
  7. Malcolm Kohler2,3
  1. 1 Epidemiology, Biostatistics and Prevention Institute, University of Zurich, Zurich, Switzerland
  2. 2 Department of Pneumology, University Hospital of Zurich, Zurich, Switzerland
  3. 3 Center of Interdisciplinary Sleep Research, University of Zurich, Zurich, Switzerland
  4. 4 Oxford Centre for Respiratory Medicine and Oxford NIHR Biomedical Research Centre, Churchill Campus, Oxford University, Oxford, UK
  1. Correspondence to Dr Malcolm Kohler, Department of Pulmonology, University Hospital Zurich, Zurich 8091, Switzerland; malcolm.kohler{at}


Background The Epworth Sleepiness Scale (ESS) is a widely used tool for assessing sleepiness in patients with obstructive sleep apnoea (OSA). We aimed to estimate the minimal important difference (MID) in patients with OSA.

Methods We used individual data from three randomised controlled trials (RCTs) in patients with OSA where the preintervention to postintervention change in ESS was used as a primary outcome. We used anchor-based linear regression and responder analysis approaches to estimate the MID. For anchors, we used the change in domains of the Functional Outcomes of Sleep Questionnaire and 36-Item Short Form Health Survey. We also used the distribution-based approaches Cohen’s effect size, SE of measurement and empirical rule effect size to support the anchor-based estimates. The final MID was determined by triangulating all estimates to a single MID.

Findings A total of 639 patients with OSA were included in our analyses across the three RCTs with a median (IQR) baseline ESS score of 10 (6–13). The median (IQR) ESS change score overall was −2 (−5 to 1). The anchor-based estimates of the MID were between −1.74 and −4.21 points and estimates from the responder analysis were between −1 and −3 points. Distribution-based estimates were smaller, ranging from −1.46 to −2.36.

Interpretation We propose an MID for the ESS of 2 points in patients with OSA with a disease severity from mild to severe. This estimate provides the means to plan trials and interpret the clinical relevance of changes in ESS.

Trial registration number Provent, NCT01332175; autoCPAP trial, NCT00280800; MOSAIC,ISRCTN (3416388).

  • sleep apnoea
View Full Text

Statistics from


  • Contributors SC, NAS and MK had full access to all data in the study. SC, NAS, KEB, JRS, AF, MAP and MK take responsibility for the integrity of the data and the accuracy of the data analysis. MAP and MK contributed to study design and obtained funding for the study. KEB, JRS and MK contributed to data collection. All authors contributed to analysis and interpretation of data. SC, AF, MAP and MK contributed to writing of the report. NAS, KEB, JRS, AF, MAP and MK contributed to critical revision.

  • Funding This work was supported by the Swiss National Science Foundation (32003B_162534) and an unrestricted grant by Bayer, Germany.

  • Competing interests MK and JRS declare advisory board fees from Bayer. Other authors have no completing interests to declare.

  • Patient consent Not required.

  • Ethics approval Research ethics committees in Zurich and Oxford.

  • Provenance and peer review Not commissioned; externally peer reviewed.

Request Permissions

If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center’s RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.