Does not delete rows containing NAs, but does add details concerning
the distribution of the response variable if options(na.detail.response=TRUE).
This na.action is primarily for use with describe.formula.
Usage
na.keep(mf)
Value
the same model frame with the "na.action" attribute
Arguments
mf
a model frame
Author
Frank Harrell
Department of Biostatistics
Vanderbilt University
fh@fharrell.com