Programming the statistical procedures from SAS

How do I nest 3 random effects in Proc Mixed?

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Posts: 1

How do I nest 3 random effects in Proc Mixed?

I'm looking for some help nesting factors.


I have the following experiment design:


Four Treatments

Four pens per treatment

Four sampled animals per pen

3 samples measured per animal, sometimes just two samples measured, at the same time point.


Previously, I have only take one sample per animal, and my mode and random statement with nesting looks something like


Model = Treatment / cl;

Random Pen(Treatment);


I'm wondering how I could work Animal into that statement.  Maybe Animal*Pen(Treatment), or Animal (Pen Treatment) ?


Furthermore, when I write out a linear model equation, I would represent the Pen(Treatment) as uij


u for random effect of j pen for i treatment


How would I properly state it if I incorporated Animal?


uija, with a for animal maybe?


I look forward to any thoughts on this would be appreciated.  Apologies if my stats language is a bit imprecise.  Too much veterinary medicine and not enough math in my life.

Posts: 188

Re: How do I nest 3 random effects in Proc Mixed?

[ Edited ]

Assuming that treatment is the only fixed effect:

class treatment pen animal;
model y = treatment; random pen(treatment)   animal*pen(treatment);

Given how SAS expands syntax, there are equivalent ways to identify a random effect in this model:

"pen(treatment)" == "pen*treatment"

"animal*pen(treatment)" == "animal*pen*treatment" == "animal(pen treatment)"


Equation-wise, something like (without going to the trouble of proper symbols)


y_ijkm = mu + t_i + p_ij + a_k(ij) + e_m(ijk)


where i indexes treatment t

j indexes pen p

k indexes animal a

m indexes sample e (residual error)


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