Premium
Internal pilot studies I: type I error rate of the naive t ‐test
Author(s) -
Wittes Janet,
Schabenberger Oliver,
Zucker David,
Brittain Erica,
Proschan Michael
Publication year - 1999
Publication title -
statistics in medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.996
H-Index - 183
eISSN - 1097-0258
pISSN - 0277-6715
DOI - 10.1002/(sici)1097-0258(19991230)18:24<3481::aid-sim301>3.0.co;2-c
Subject(s) - type i and type ii errors , sample size determination , statistics , mathematics , variance (accounting) , interim , computer science , accounting , archaeology , business , history
Abstract When sample size is recalculated using unblinded interim data, use of the usual t ‐test at the end of a study may lead to an elevated type I error rate. This paper describes a numerical quadrature investigation to calculate the true probability of rejection as a function of the time of the recalculation, the magnitude of the detectable treatment effect, and the ratio of the guessed to the true variance. We consider both ‘restricted’ designs, those that require final sample size at least as large as the originally calculated size, and ‘unrestricted’ designs, those that permit smaller final sample sizes than originally calculated. Our results indicate that the bias in the type I error rate is often negligible, especially in restricted designs. Some sets of parameters, however, induce non‐trivial bias in the unrestricted design. Copyright © 1999 John Wiley & Sons, Ltd.