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.