R: I-Spline Basis for Polynomial Splines

iSpline {splines2}

R Documentation

I-Spline Basis for Polynomial Splines

Description

Generates the I-spline (integral of M-spline) basis matrix for a polynomial spline or the corresponding derivatives of given order.

Usage

iSpline(
  x,
  df = NULL,
  knots = NULL,
  degree = 3L,
  intercept = TRUE,
  Boundary.knots = NULL,
  derivs = 0L,
  ...
)

Arguments

`x`	The predictor variable. Missing values are allowed and will be returned as they are.
`df`	Degree of freedom that equals to the column number of the returned matrix. One can specify `df` rather than `knots`, then the function chooses `df - degree - as.integer(intercept)` internal knots at suitable quantiles of `x` ignoring missing values and those `x` outside of the boundary. If internal knots are specified via `knots`, the specified `df` will be ignored.
`knots`	The internal breakpoints that define the splines. The default is `NULL`, which results in a basis for ordinary polynomial regression. Typical values are the mean or median for one knot, quantiles for more knots.
`degree`	The degree of I-spline defined to be the degree of the associated M-spline instead of actual polynomial degree. For example, I-spline basis of degree 2 is defined as the integral of associated M-spline basis of degree 2.
`intercept`	If `TRUE` by default, all of the spline basis functions are returned. Notice that when using I-Spline for monotonic regression, `intercept = TRUE` should be set even when an intercept term is considered additional to the spline basis functions.
`Boundary.knots`	Boundary points at which to anchor the splines. By default, they are the range of `x` excluding `NA`. If both `knots` and `Boundary.knots` are supplied, the basis parameters do not depend on `x`. Data can extend beyond `Boundary.knots`.
`derivs`	A non-negative integer specifying the order of derivatives of I-splines.
`...`	Optional arguments that are not used.

Details

It is an implementation of the close form I-spline basis based on the recursion formula given by Ramsay (1988).

Value

A numeric matrix of length(x) rows and df columns if df is specified or length(knots) + degree + as.integer(intercept) columns if knots are specified instead. Attributes that correspond to the arguments specified are returned mainly for other functions in this package.

References

Ramsay, J. O. (1988). Monotone regression splines in action. Statistical Science, 3(4), 425–441.

Examples

library(splines2)

## Example given in the reference paper by Ramsay (1988)
x <- seq.int(0, 1, by = 0.01)
knots <- c(0.3, 0.5, 0.6)
isMat <- iSpline(x, knots = knots, degree = 2)

op <- par(mar = c(2.5, 2.5, 0.2, 0.1), mgp = c(1.5, 0.5, 0))
matplot(x, isMat, type = "l", ylab = "I-spline basis")
abline(v = knots, lty = 2, col = "gray")

## reset to previous plotting settings
par(op)

## the derivative of I-splines is M-spline
msMat1 <- iSpline(x, knots = knots, degree = 2, derivs = 1)
msMat2 <- mSpline(x, knots = knots, degree = 2, intercept = TRUE)
stopifnot(all.equal(msMat1, msMat2))

[Package splines2 version 0.4.2 Index]