```Date: Tue, 14 Nov 2006 16:27:47 -0600 Reply-To: "Swank, Paul R" Sender: "SAS(r) Discussion" From: "Swank, Paul R" Subject: Re: question on Comparing two regression lines Comments: To: jn mao Content-Type: text/plain; charset="us-ascii" The interaction is a test of equal slopes between groups and the fact that it is not significant is good since the compariosn of the groups across doses can proceed once the homogeneity of regression slopes seems tenable. Now drop the interaction and run the model with just X and group. This is akin to analysis of covariance. if the group effect is significant, that means the groups do differ and the lack of interaction means that the group difference does not depend on the dose. Dose may still be related to the outcome. Of course, statistical tests are no guarantee of difference, but an indication that there is evidence to support the research hypothesis. And all this presupposes that your data are reasonable for this analysis. Paul R. Swank, Ph.D. Professor, Developmental Pediatrics Director of Research, University of Texas Health Science Center at Houston -----Original Message----- From: SAS(r) Discussion [mailto:SAS-L@LISTSERV.UGA.EDU] On Behalf Of jn mao Sent: Tuesday, November 14, 2006 3:29 PM To: SAS-L@LISTSERV.UGA.EDU Subject: question on Comparing two regression lines Hello SASLs, I have a continuous Y measured at 3 doses of drug A given (X) between two groups. My boss asked me to compare if overall Y is significant difference with 3 doses between 2 groups. I did 'proc glm' to compare the regression line of Y with X (x=1, or 2 or 3 for 3 doses) between two groups. The interaction p value is >0.05. I did conclude that overall Y is not significantly different with increasing dose of drug A between two groups. Is that conclusion right? As my boss asked that the interaction is not significant only means that the two regression lines are parallel, but the difference between two parallel lines could be significant. He said there must be some else analysis that could answer the question of overall Y is different between two groups. Could someone help me on this question? Thanks much. Jane --------------------------------- Cheap Talk? Check out Yahoo! Messenger's low PC-to-Phone call rates. ```

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