PERFORMANCE OF A PRELIMINARY TEST OF COMPARABILITY IN OBSERVATIONAL STUDIES.
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)
Document Details
- Document Type
- Technical Report
- Publication Date
- Feb 04, 1970
- Accession Number
- AD0702056
Entities
People
- W. G. Cochran
Organizations
- Harvard University