Information from the action which was applied to object if
NAs were handled specially, or NULL.
Details
na.action is a generic function, and na.action.default its
default method. The latter extracts the "na.action" component
of a list if present, otherwise the "na.action" attribute. When model.frame is called, it records any information
on NA handling in a "na.action" attribute. Most
model-fitting functions return this as a component of their result.
References
Chambers, J. M. and Hastie, T. J. (1992)
Statistical Models in S.
Wadsworth & Brooks/Cole.