Parameters | Expression Parameters | Unbounded Variance Component Estimates

Unbounded Variance Component Estimates
Check this option to allow variance component estimates to be negative for the random effects in your model .
This condition is appropriate when negative correlations between observations are reasonable, and can make fixed effect tests less biased.
Caution : Selecting this option can prove considerably more computationally intensive than the default method, which constrains variance component estimates to be nonnegative.
To Specify this Option:
Refer to the SAS documentation for PROC MIXED for more information.