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\usage{
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\usage{
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interaction(\dots, drop = FALSE)
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interaction(\dots, drop = FALSE)
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}
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}
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\alias{interaction}
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\alias{interaction}
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\arguments{
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\arguments{
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  \item{\dots}{The factors for which interaction is to be computed, or a
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  \item{\dots}{the factors for which interaction is to be computed, or a
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    single list giving those factors.}
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    single list giving those factors.}
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  \item{drop}{If \code{drop} is \code{TRUE}, empty factor levels
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  \item{drop}{if \code{drop} is \code{TRUE}, empty factor levels
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    are dropped from the result. The default is to retain all
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    are dropped from the result.  The default is to retain all
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    factor levels.}
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    factor levels.}
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}
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}
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\description{
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\description{
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  \code{interaction} computes a factor which represents the interaction
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  \code{interaction} computes a factor which represents the interaction
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  of the given factors.  The result of \code{interaction} is always unordered.
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  of the given factors.  The result of \code{interaction} is always unordered.
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}
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}
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\value{
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\value{
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  A factor which represents the interaction of the given factors.
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  A factor which represents the interaction of the given factors.
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  The levels are labelled as the levels of the individual factors joined
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  by \code{.}.
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}
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}
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\references{
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\references{
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  Chambers, J. M. and Hastie, T. J. (1992)
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  Chambers, J. M. and Hastie, T. J. (1992)
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  \emph{Statistical Models in S}.
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  \emph{Statistical Models in S}.
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  Wadsworth \& Brooks/Cole.
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  Wadsworth \& Brooks/Cole.
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}
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}
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\seealso{
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\seealso{\code{\link{factor}}.}%% even more ":" for factors!
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  \code{\link{factor}}, \code{\link{:}}
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}
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\examples{
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\examples{
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a <- gl(2, 2, 8)
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a <- gl(2, 2, 8)
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b <- gl(2, 4, 8)
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b <- gl(2, 4, 8)
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interaction(a, b)
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interaction(a, b)