```Date: Fri, 30 May 2008 19:19:21 -0500 Reply-To: paul@WUBIOS.WUSTL.EDU Sender: "SAS(r) Discussion" From: Paul Thompson Subject: Re: modelling lactation curves Comments: To: nchapinal@YAHOO.COM In-Reply-To: <0fdf7a2c-f2f5-4422-80d9-c5a27386acd3@s21g2000prm.googlegroups.com> Content-Type: text/plain; charset=ISO-8859-1; DelSp="Yes"; format="flowed" Quoting nchapinal@YAHOO.COM: 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 cday=day-150 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. > Hi, > > 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; > run; > > 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? > > Thanks! > ```

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