Date: Fri, 30 May 2008 19:19:21 -0500
Sender: "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From: Paul Thompson <paul@WUBIOS.WUSTL.EDU>
Subject: Re: modelling lactation curves
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Make it a quadratic model.
1) Center day. I assume that "day" is 0 at calving, and goes for 300
days or so. To center it, subtract the mean day. Call that
Of course, the intercept is now the middle of the measurement period.
2) Compute cday-squared. Call it cday2 or cdaysq.
Model production=cday cdaysq treatment cday*treatment cdaysq*treatment;
If you don't center day, day and daysq are highly collinear, and the
estimates have high standard errors.
> milk production follows a curve, as some of you may already now. When
> a high producer cow calves, the milk production per day is aroung 10
> kg a day, but increases very quickly getting soon a peak (even more
> than 40 kg/day) and then it starts decreasing slowly.
> I have seen paper that compare the milk production between two
> treatments by using PROC MIXED and REPEATED.
> For instance:
> proc mixed;
> class cow treatment;
> model production = treatment day treatment*day;
> repeated /subject=cow type=cs; *they usually try different types to
> see which one fits better;
> However, to me, that is not totally sound, since milk production is
> not linear.
> Is there any way to overcome this problem? Is it going to make any
> difference in the results?