Estimate variance of nondecision density
getSter(res)An object of class D*M.
The object res can either be output from estDstarM or output from estND.
if the former is supplied, getSter attempts to calculate the variance of the
nondecision distribution by subtracting the variance of the model distribution from the
variance of the data distribution. If the latter is supplied, the variance is calculated by
integrating the nondecision distribution.