Date: Tue, 27 Jan 2004 14:02:51 -0500 Peter Flom "SAS(r) Discussion" Peter Flom Re: build a model with only the significant factorsandinteractions To: knada@ACESAG.AUBURN.EDU text/plain; charset=US-ASCII

Well that's why I said it's ALMOST never a good idea.....:-)

However, wouldn't the situation below be better handled using a hierarchical linear model with PROC MIXED ? Seems like either a two level model (cows nested in herds) or possibly a 3 level model, if it's the same cows being measured at different ages.

Peter

Peter L. Flom, PhD Assistant Director, Statistics and Data Analysis Core Center for Drug Use and HIV Research National Development and Research Institutes 71 W. 23rd St www.peterflom.com New York, NY 10010 (212) 845-4485 (voice) (917) 438-0894 (fax)

There are SOME unique situations in Animal breeding where we often re-code and fit "interaction terms" as main effect ( sometime as fixed or random) in estimating parameters. Look at a simple classic situation where if I want to estimate the AGE effect of cows (2, 3, 4,5... yr. olds) on milk production records collected from different herds (#100, 101, 102..) in different years ( 90, 91 ,92..) and different seasons ( fall=1, winter=2, spring=3 and summer=4). We are aware that because of the large variations in management levels in herds, good & bad years for pature/feeding, season (cold, hot) these effect would be be significant. Our objective is to get a best estimate for Age effect on milk production. We want to fit a model that accounts for all the above effects and produce the best estimate for Age.

So, the herd -Year-season (the interaction) could be re-coded in some fashion as (herd*1000000 + year*100 + season ), gives us a unique code for that interaction. example : herd #101, yr= 91, season=1 would be 10109101, is the effect for that combination. Here I am not interested in that interaction, I knew that will be significant, all I want is to fit the efficient model to get the best estimate for Age effect on milk production.

Just to give some thoughts on interactions as main effect.