Assessing the effect of a treatment when subjects are growing at different rates.
The analysis of covariance is often used in the context of premeasure/postmeasure designs to compare treatment and control groups in both randomized [1] and nonrandomized [2] studies. The intent is to adjust the difference between the changes in the 2 groups for any difference which might exist at baseline, i.e., for any difference between the premeasures in the 2 groups. An important assumption underlying the use of the analysis of covariance is that the slopes of the lines for the regression of the postmeasure on the premeasure in the 2 groups are equal. In this paper we describe a program which can be used to test the hypothesis of equal slopes; and performs an alternative analysis which does not depend on this assumption. This is done in the context of comparing treatment and control groups with respect to a measurement subject to natural maturation as in [3]. Equal slopes in this context means equal growth rates; unequal slopes implies that the 2 groups are growing at different rates. The method, known as the Johnson-Neyman procedure [4] is, however, more general than this, and can be used in any two-sample comparison where an alternative to the usual analysis of covariance is deemed appropriate. The procedure identifies a 'region of significance' which is especially useful in practice. This region consists of a set of values of the premeasure for which the treatment and the control groups are significantly different with respect to the postmeasure.
Kowalski
C J
CJ
Department of Biologic and Materials Sciences, Dental School, University of Michigan, Ann Arbor 48109.
Schneiderman
E D
ED
Willis
S M
SM
D000704:Analysis of Variance; D000818:Animals; D001835:Body Weight / Q000187:drug effects; D003198:Computer Simulation; D006128:Growth / Q000187:drug effects*; D008432:Mathematical Computing; D008838:Microcomputers; D008954:Models, Biological; D015233:Models, Statistical; D051381:Rats; D012044:Regression Analysis; D012107:Research Design; D012984:Software; D013889:Thiouracil / Q000494:pharmacology
D013889:Thiouracil