Generic function for extracting an estimator for the bread of
sandwiches.
Usage
bread(x, ...)
Arguments
x
a fitted model object.
...
arguments passed to methods.
Value
A matrix containing an estimator for the expectation of the negative
derivative of the estimating functions, usually the Hessian.
Typically, this should be an $k \times k$ matrix corresponding
to $k$ parameters. The rows and columns should be named
as in coef or terms, respectively.
References
Zeileis A (2006), Object-oriented Computation of Sandwich Estimators.
Journal of Statistical Software, 16(9), 1--16.
URL http://www.jstatsoft.org/v16/i09/.