Users' Guides to the Medical Literature
Guyatt G, Rennie D, Meade MO, Cook DJ
Part B Therapy
Chapter 10.3. What Determines the Width of the Confidence Interval?
Sample Size Does Not Determine the Width of the Confidence Interval
confidence interval, precision of effects, sample size, study design
"Clinicians sometimes equate the size of a randomized
trial or the number of participants in a trial
of therapy with its precision, and thus the width of the confidence interval (CI).
This chapter deals with issues of precision and the resulting confidence
in estimates of treatment effects in studies in which the outcomes are dichotomous (yes/no)
events such as death, stroke, or myocardial infarction. As it turns
out, the number of patients is a secondary determinant of our confidence
in estimates of reduction in adverse outcomes associated with an experimental intervention...."
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