07-15-2012 03:20 AM
I have a data set where I have a dependent variable Y , a treatment variable (Trt) and set of random factors z1(10 levels), z2(2 levels) and z3(2 levels) such that z3 is nested in z2 and z2 is nested in z1 I am using proc mixed to analyze this data. Code below :
proc mixed method=REML;
class trt z1 z2 z3;
model y = trt;
random z1 z2(z1) z3(z1 z2);
The problem is the covariance estimate for z2 comes out as zero. I am wondering why I am getting such an estimate. Any help/insight would be appreciated.
PS: I have also tried other methods i.e. MIVQUE0 , type 1 and type 2 and ML
07-16-2012 08:39 AM
It most likely means that there is insufficient variability remaining after fitting z1 to get a positive estimate with the data you have in hand. The only real solution is the most expensive one--obtain more data.