"Non-positive definite approximate variance-covariance" output

Hi all,

I was in a discussion with my advisor today, and the issue of "non-positive
definite approximate variance-covariance" errors in R when using the lme()
function arose.

Depending on the varFunc specification that I use, I receive the
"non-positive definite approximate variance-covariance" output when calling
myModel$apVar -- however, there are standard errors returned for my model
coefficients in the model summary.

My understanding is that if the variance-covariance cannot be approximated,
neither should the standard errors... how is the lme() function returning
standard errors without a variance-covariance approximation?

Many thanks,
Jacob

Jacob J. Bukoski
Master of Environmental Science Candidate, 2016
School of Forestry and Environmental Studies, Yale University
jbukoski1 at gmail.com | jacob.bukoski at yale.edu | LinkedIn
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