RT Book, Section A1 Brożek, Jan A1 Falavigna, Maicon A2 Guyatt, Gordon A2 Rennie, Drummond A2 Meade, Maureen O. A2 Cook, Deborah J. SR Print(0) ID 1183876358 T1 What Determines the Width of the Confidence Interval? T2 Users' Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice, 3rd ed YR 2015 FD 2015 PB McGraw-Hill Education PP New York, NY SN 978-0-07-179071-0 LK jamaevidence.mhmedical.com/content.aspx?aid=1183876358 RD 2024/03/29 AB Clinicians sometimes equate the size of a study or the number of participants in a study with the width of the confidence interval (CI) and thus with its precision. This chapter deals with issues of precision and the resulting CIs associated with treatment effects on dichotomous (yes/no) outcomes, such as death, stroke, or myocardial infarction. As it turns out, for the relative measures of effect sizes (eg, relative risk [RR] or relative risk reduction [RRR]), the number of patients in a study is a secondary determinant of the width of a CI, with the primary determinant being the absolute number of events.