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% File src/library/stats/man/makepredictcall.Rd
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% Part of the R package, https://www.R-project.org
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% Copyright 1995-2018 R Core Team
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% Distributed under GPL 2 or later
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\name{makepredictcall}
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\alias{makepredictcall}
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\alias{makepredictcall.default}
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\alias{SafePrediction}
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\title{Utility Function for Safe Prediction}
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\description{
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A utility to help \code{\link{model.frame.default}} create the right
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matrices when predicting from models with terms like (univariate)
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\code{poly} or \code{ns}.
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}
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\usage{
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makepredictcall(var, call)
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}
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\arguments{
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\item{var}{A variable.}
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\item{call}{The term in the formula, as a call.}
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}
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\details{
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This is a generic function with methods for \code{poly}, \code{bs} and
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\code{ns}: the default method handles \code{scale}. If
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\code{model.frame.default} encounters such a term when
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creating a model frame, it modifies the \code{predvars} attribute of
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the terms supplied by replacing the term with one which will work for
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predicting new data. For example \code{makepredictcall.ns} adds
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arguments for the knots and intercept.
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To make use of this, have your model-fitting function return the
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\code{terms} attribute of the model frame, or copy the \code{predvars}
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attribute of the \code{terms} attribute of the model frame to your
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\code{terms} object.
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To extend this, make sure the term creates variables with a class,
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and write a suitable method for that class.
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}
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\value{
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A replacement for \code{call} for the \code{predvars} attribute of
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the terms.
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}
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\seealso{
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\code{\link{model.frame}}, \code{\link{poly}}, \code{\link{scale}};
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\code{\link{bs}} and \code{\link{ns}} in package \pkg{splines}.
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\code{\link{cars}} for an example of prediction from a polynomial fit.
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}
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\examples{
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require(graphics)
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## using poly: this did not work in R < 1.5.0
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fm <- lm(weight ~ poly(height, 2), data = women)
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plot(women, xlab = "Height (in)", ylab = "Weight (lb)")
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ht <- seq(57, 73, length.out = 200)
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nD <- data.frame(height = ht)
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pfm <- predict(fm, nD)
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lines(ht, pfm)
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pf2 <- predict(update(fm, ~ stats::poly(height, 2)), nD)
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stopifnot(all.equal(pfm, pf2)) ## was off (rel.diff. 0.0766) in R <= 3.5.0
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## see also example(cars)
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## see bs and ns for spline examples.
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}
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\keyword{models}
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