  While robust standard errors and related facilities are available in R for
many types of statistical models, the facilities are notably lacking for models
estimated via lme4. This is because the necessary statistical output, including
the Hessian and casewise gradient of random effect parameters, is not
immediately available from lme4 and is not trivial to obtain. In this article,
we supply and describe two new functions to obtain this output from Gaussian
mixed models: estfun.lmerMod() and vcov.full.lmerMod(). We discuss the
theoretical results implemented in the code, focusing on calculation of robust
standard errors via package sandwich. We also use the Sleepstudy data to
illustrate the code and compare it to a benchmark from package lavaan.
