cluster: a variable indicating groupings. ‘cluster’ may be any type of vector (factor, character, integer). NAs are not allowed. Unique values of ‘cluster’ indicate possibly correlated groupings of observations. Note the data used in the fit and stored in ‘fit$x’ and ‘fit$y’ may have had observations containing missing values deleted. It is assumed that if any NAs were removed during the original model fitting, an ‘naresid’ function exists to restore NAs so that the rows of the score matrix coincide with ‘cluster’. If ‘cluster’ is omitted, it defaults to the integers 1,2,...,n to obtain the "sandwich" robust covariance matrix estimate.