Each variable named in the expression after the
| operator on
the right hand side of
form is evaluated in
more than one variable is indicated in
level they are combined
into a data frame; else the selected variable is returned as a vector.
When multiple grouping levels are defined in
level > 1, the levels of the returned factor are obtained by
pasting together the levels of the grouping factors of level greater
or equal to
level, to ensure their uniqueness.
an object inheriting from class
an optional formula with a conditioning expression on its
right hand side (i.e. an expression involving the
a positive integer vector with the level(s) of grouping to be used when multiple nested levels of grouping are present. Defaults to all levels of nesting.
character, the separator to use between group levels when
multiple levels are collapsed. The default is
either a data frame with columns given by the grouping factors
level, from outer to inner, or, when a single
level is requested, a factor representing the selected grouping
José Pinheiro and Douglas Bates [email protected]
Pinheiro, J.C., and Bates, D.M. (2000) "Mixed-Effects Models in S and S-PLUS", Springer, esp. pp. 100, 461.
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