Date: Fri, 30 May 2008 21:16:12 -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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Quoting Kanagasabai Nadarajah <nadarka@AUBURN.EDU>:
> Fitting various type of models and procedures to predict lactation
> curves have been extensively studied by animal scientists.
> This paper compares many options and covers other work too.
> Models for Estimating Typical Lactation Curves in Dairy Cattle
> C.U. Leon-Velarde,, I. McMillan, R.D. Gentry and J.W. Wilton
> J. Anim. Breed. Genet. (1995) 112:333-340
THanks for this. It really did sound way too obvious that no one
would have tried a quadratic model before. Plenty of sharp people in
animal sciences. That is the real discipline in which mixed models of
certain types were first developed.
> Nada K. Nadarajah, PhD
> Senior Research Fellow
> Dept. of Animal Sciences.
> Auburn University.
> Auburn, AL 36849-5415
> Tel: 334-844-1502
> Fax: 334-844-1519
> URL: http://www.ag.auburn.edu/ansc/staff/nada.html
>>>> Peter Flom <email@example.com> 05/30/08 7:48 PM >>>
>> 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?
> Paul made the excellent suggestion of centering and then fitting a
> quadratic. However, even this may fit your curve adequately. With
> independent data, I'd suggest a spline, but, AFAIK, this isn't available
> in MIXED. You might be able to do something in NLMIXED (and Dale may be
> chiming in with much better advice), but how many time points do you
> typically have? If it's more than (say) 20 per cow, have you considered
> some form of time series analysis?
> Peter L. Flom, PhD
> Statistical Consultant
> www DOT peterflom DOT com