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 t-test of (x1-x2) is made. If t is not significant, (y1-y2) is used as an estimate of the population difference delta y; if t is significant, (y1-y2) 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