Date:         Fri, 30 May 2008 16:08:47 -0700
Reply-To:     nchapinal@YAHOO.COM
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         nchapinal@YAHOO.COM
Organization: http://groups.google.com
Subject:      modelling lactation curves
Comments: To: sas-l@uga.edu
Content-Type: text/plain; charset=ISO-8859-1

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!