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Date:         Thu, 17 Jan 2008 14:29:15 -0600
Reply-To:     "data _null_," <datanull@GMAIL.COM>
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         "data _null_," <datanull@GMAIL.COM>
Subject:      Re: Weird result in PROC GLM
Comments: To: Peter Flom <peterflomconsulting@mindspring.com>
In-Reply-To:  <7407591.1200594296930.JavaMail.root@mswamui-chipeau.atl.sa.earthlink.net>
Content-Type: text/plain; charset=ISO-8859-1

Perhaps this example will help. The means are depend on the coding 0 vs -1. In the REG the intercepts are different. The intercept and mean are related, yes?

data test; do coding = -1,0; do year = 1989 to 2007; LateYear =ifn(year>1997,1,coding); earlyYear=ifn(year<1998,1,coding); output; end; end; run; proc print; run; proc means; class coding; run; proc reg noprint outest=est; by coding notsorted; late:model year = lateYear; early:model year = earlyYear; run; proc print; run;

On Jan 17, 2008 12:24 PM, Peter Flom <peterflomconsulting@mindspring.com> wrote: > Hello > > We have a data set with 3 variables: Year, percent HIV and research estimate. > > At the request of our boss, we converted year into a dichotomous variable (not my choice of analysis...) > > My colleague did this two ways: > > 1) Lateyear = 1 if year > 1997, else 0 > and > 2) earlyyear = 1 if year < 1998, else 0 > > as a learning exercise we then ran both > > PROC GLM; > MODEL RESEARCH_ESTIMATE = PERCENT_HIV LATEYEAR PERCENT_HIV*LATEYEAR; > RUN; > > and > > PROC GLM; > MODEL RESEARCH_ESTIMATE = PERCENT_HIV EARLYYEAR PERCENT_HIV*EARLYYEAR; > RUN; > > I expected these to be identical, except for change of sign. Indeed, most of the results are identical, R-square, type I SS, F value etc.. But the TYPE III SS results for PERCENT_HIV are different by a factor of 8; the parameter estimate for the intercept is different, and the parameter estimate for PERCENT_HIV is different by a small amount, and the SE for the intercept and PERCENT_HIV are different. > > Any ideas how this could happen? > > Thanks > > Peter >


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