% Generated by roxygen2: do not edit by hand
% Please edit documentation in R/contrast.R
\name{contrast}
\alias{contrast}
\alias{contrast.emmGrid}
\alias{pairs.emmGrid}
\alias{coef.emmGrid}
\title{Contrasts and linear functions of EMMs}
\usage{
contrast(object, ...)

\method{contrast}{emmGrid}(object, method = "eff", interaction = FALSE, by,
  offset = NULL, scale = NULL, name = "contrast",
  options = get_emm_option("contrast"), type, adjust, simple,
  combine = FALSE, ratios = TRUE, parens, ...)

\method{pairs}{emmGrid}(x, reverse = FALSE, ...)

\method{coef}{emmGrid}(object, ...)
}
\arguments{
\item{object}{An object of class \code{emmGrid}}

\item{...}{Additional arguments passed to other methods}

\item{method}{Character value giving the root name of a contrast method (e.g.
\code{"pairwise"} -- see \link{emmc-functions}). Alternatively, a function
of the same form, or a named \code{list} of coefficients (for a contrast or
linear function) that must each conform to the number of results in each
\code{by} group. In a multi-factor situation, the factor levels are
combined and treated like a single factor.}

\item{interaction}{Character vector, logical value, or list. If this is specified,
\code{method} is ignored. See the \dQuote{Interaction contrasts} section
below for details.}

\item{by}{Character names of variable(s) to be used for ``by'' groups. The
contrasts or joint tests will be evaluated separately for each combination
of these variables. If \code{object} was created with by groups, those are
used unless overridden. Use \code{by = NULL} to use no by groups at all.}

\item{offset, scale}{Numeric vectors of the same length as each \code{by} group.
The \code{scale} values, if supplied, multiply their respective linear estimates, and
any \code{offset} values are added. Scalar values are also allowed. 
(These arguments are ignored when \code{interaction} is specified.)}

\item{name}{Character name to use to override the default label for contrasts
used in table headings or subsequent contrasts of the returned object.}

\item{options}{If non-\code{NULL}, a named \code{list} of arguments to pass
to \code{\link{update.emmGrid}}, just after the object is constructed.}

\item{type}{Character: prediction type (e.g., \code{"response"}) -- added to
\code{options}}

\item{adjust}{Character: adjustment method (e.g., \code{"bonferroni"}) --
added to \code{options}}

\item{simple}{Character vector or list: Specify the factor(s) \emph{not} in
\code{by}, or a list thereof. See the section below on simple contrasts.}

\item{combine}{Logical value that determines what is returned when
\code{simple} is a list. See the section on simple contrasts.}

\item{ratios}{Logical value determining how log and logit transforms are
handled. These transformations are exceptional cases in that there is a
valid way to back-transform contrasts: differences of logs are logs of
ratios, and differences of logits are odds ratios. If \code{ratios = TRUE}
and summarized with \code{type = "response"}, \code{contrast} results are
back-transformed to ratios whenever we have true contrasts (coefficients
sum to zero). For other transformations, there is no natural way to
back-transform contrasts, so even when summarized with \code{type = "response"},
contrasts are computed and displayed on the linear-predictor scale. Similarly, 
if \code{ratios = FALSE}, log and logit transforms are treated in the same way as
any other transformation.}

\item{parens}{character or \code{NULL}. If a character value, the labels for levels
being contrasted are parenthesized if they match the regular expression in 
\code{parens[1]} (via \code{\link{grep}}). The default is \code{emm_option("parens")}.
Optionally, \code{parens} may contain second and third elements specifying
what to use for left and right parentheses (default \code{"("} and \code{")"}).
Specify \code{parens = NULL} or \code{parens = "a^"} (which won't match anything)
to disable all parenthesization.}

\item{x}{An \code{emmGrid} object}

\item{reverse}{Logical value - determines whether to use \code{"pairwise"} (if \code{TRUE}) or \code{"revpairwise"} (if \code{FALSE}).}
}
\value{
\code{contrast} and \code{pairs} return an object of class
  \code{emmGrid}. Its grid will correspond to the levels of the contrasts and
  any \code{by} variables. The exception is that an \code{\link{emm_list}}
  object is returned if \code{simple} is a list and \code{complete} is
  \code{FALSE}.

\code{coef} returns a \code{data.frame} containing the object's grid, along with columns named \code{c.1, c.2, ...} containing the contrast coefficients. If
}
\description{
These methods provide for follow-up analyses of \code{emmGrid} objects:
Contrasts, pairwise comparisons, tests, and confidence intervals. They may
also be used to compute arbitrary linear functions of predictions or EMMs.
}
\note{
When \code{object} has a nesting structure (this can be seen via
  \code{str(object)}), then any grouping factors involved are forced into
  service as \code{by} variables, and the contrasts are thus computed
  separately in each nest. This in turn may lead to an irregular grid in the
  returned \code{emmGrid} object, which may not be valid for subsequent
  \code{emmeans} calls.
}
\section{Pairs method}{
 The call \code{pairs(object)} is equivalent to
  \code{contrast(object, method = "pairwise")}; and \code{pairs(object,
  reverse = TRUE)} is the same as \code{contrast(object, method =
  "revpairwise")}.
}

\section{Interaction contrasts}{
 When \code{interaction} is specified,
  interaction contrasts are computed. Specifically contrasts are generated
  for each factor separately, one at a time; and these contrasts are applied
  to the object (the first time around) or to the previous result
  (subsequently). (Any factors specified in \code{by} are skipped.) The final
  result comprises contrasts of contrasts, or, equivalently, products of
  contrasts for the factors involved. Any named elements of \code{interaction}
  are assigned to contrast methods; others are assigned in order of
  appearance in \code{object@levels}. The contrast factors in the resulting 
  \code{emmGrid} object are ordered the same as in \code{interaction}.
  
  \code{interaction} may be a character vector or list of valid contrast
  methods (as documented for the \code{method} argument). If the vector or
  list is shorter than the number needed, it is recycled. Alternatively, if
  the user specifies \code{contrast = TRUE}, the contrast specified in
  \code{method} is used for all factors involved.
}

\section{Simple contrasts}{

  \code{simple} is essentially the complement of \code{by}: When
  \code{simple} is a character vector, \code{by} is set to all the factors in
  the grid \emph{except} those in \code{simple}. If \code{simple} is a list,
  each element is used in turn as \code{simple}, and assembled in an
  \code{"emm_list"}. To generate \emph{all} simple main effects, use
  \code{simple = "each"} (this works unless there actually is a factor named
  \code{"each"}). Note that a non-missing \code{simple} will cause \code{by}
  to be ignored.
  
  Ordinarily, when \code{simple} is a list or \code{"each"}, the return value
  is an \code{\link{emm_list}} object with each entry in correspondence with
  the entries of \code{simple}. However, with \code{combine = TRUE}, the
  elements are all combined into one family of contrasts in a single
  \code{\link[=emmGrid-class]{emmGrid}} object using
  \code{\link{rbind.emmGrid}}.. In that case, the \code{adjust} argument sets
  the adjustment method for the combined set of contrasts.
}

\examples{
warp.lm <- lm(breaks ~ wool*tension, data = warpbreaks)
warp.emm <- emmeans(warp.lm, ~ tension | wool)
contrast(warp.emm, "poly")    # inherits 'by = "wool"' from warp.emm
pairs(warp.emm)               # ditto
contrast(warp.emm, "eff", by = NULL)  # contrasts of the 6 factor combs
pairs(warp.emm, simple = "wool") # same as pairs(warp.emm, by = "tension")

# Do all "simple" comparisons, combined into one family
pairs(warp.emm, simple = "each", combine = TRUE)

\dontrun{

## Note that the following are NOT the same:
contrast(warp.emm, simple = c("wool", "tension"))
contrast(warp.emm, simple = list("wool", "tension"))
## The first generates contrasts for combinations of wool and tension
##   (same as by = NULL)
## The second generates contrasts for wool by tension, and for 
##   tension by wool, respectively.
}

# An interaction contrast for tension:wool
tw.emm <- contrast(warp.emm, interaction = c(tension = "poly", wool = "consec"), 
                   by = NULL)
tw.emm          # see the estimates
coef(tw.emm)    # see the contrast coefficients

# Use of scale and offset
#   an unusual use of the famous stack-loss data...
mod <- lm(Water.Temp ~ poly(stack.loss, degree = 2), data = stackloss)
(emm <- emmeans(mod, "stack.loss", at = list(stack.loss = 10 * (1:4))))
# Convert results from Celsius to Fahrenheit:
confint(contrast(emm, "identity", scale = 9/5, offset = 32))

}
