
Accession Number : AD0702056
Title : PERFORMANCE OF A PRELIMINARY TEST OF COMPARABILITY IN OBSERVATIONAL STUDIES.
Descriptive Note : Technical rept.,
Corporate Author : HARVARD UNIV CAMBRIDGE MASS DEPT OF STATISTICS
Personal Author(s) : Cochran,W. G.
Report Date : 04 FEB 1970
Pagination or Media Count : 17
Abstract : The note studies the performance of a preliminary test of significance sometimes used in observational studies. The means y1, y2 of a response variable are being compared in two independent samples of size n. It is feared that the comparison may be biased because a confounding variable x may have different levels (means) in the two populations. A ttest of (x1x2) is made. If t is not significant, (y1y2) is used as an estimate of the population difference delta y; if t is significant, (y1y2) is adjusted by an analysis of covariance. This procedure is found to reduce the remaining bias in the estimate of delta y to a value that does not seriously disturb tests of significance or confidence probabilities based on this estimate. If the estimation of small values of delta y is important, however, the remaining bias may still be a sizeable percentage of delta y, except with large samples. (Author)
Descriptors : (*STATISTICAL DATA, CORRELATION TECHNIQUES), (*STATISTICAL TESTS, EXPERIMENTAL DESIGN), ANALYSIS OF VARIANCE, DECISION THEORY, CONFIDENCE LIMITS, SAMPLING
Subject Categories : Statistics and Probability
Distribution Statement : APPROVED FOR PUBLIC RELEASE