MethodsEfficient Sampling in Unmatched Case–Control Studies When the Total Number of Cases and Controls Is FixedGroenwold, Rolf H. H.; van Smeden, MaartenAuthor Information From the Julius Center for Health Sciences and Primary Care, University Medical Center Utrecht, Utrecht, the Netherlands. Submitted 16 November 2016; accepted 30 June 2017. Availability of data: Not applicable. Supported by Netherlands Organisation for Scientific Research (project 917.16.430). The authors report no conflicts of interest. Correspondence: Rolf H. H. Groenwold, Julius Center for Health Sciences and Primary Care, University Medical Center Utrecht, PO Box 85500, 3508 GA Utrecht, the Netherlands. E-mail: [email protected]. Epidemiology: November 2017 - Volume 28 - Issue 6 - p 834-837 doi: 10.1097/EDE.0000000000000710 Buy Metrics Abstract In the case–control design, statistical efficiency is balanced with practical efficiency. In an unmatched case–control study of a binary exposure, in which the expected odds ratio deviates from the null (i.e., no effect) and the sum of the number of cases and controls is fixed, 1:1 sampling of cases and controls may yield suboptimal statistical efficiency. This holds especially for situations where the exposure is rare and for stronger relationships between the exposure and the outcome under study. The equations presented here may provide researchers with a simple and efficient strategy for sampling cases and controls when designing an unmatched case–control study for such a situation. Copyright © 2017 Wolters Kluwer Health, Inc. All rights reserved.