mars.to.earth {earth}R Documentation

Convert a "mars" object to an "earth" object

Description

Convert a mars object to an earth object

Usage

mars.to.earth(object)

Arguments

object A "mars" object, created using mars in the mda package.

Value

The value is the same format as that returned by earth but with skeletal versions of rss.per.subset, gcv.per.subset, and prune.terms.

You can fully initialise these components by calling update.earth after mars.to.earth, but if you do this selected.terms may change. However with pmethod="backward" a change is unlikely — selected.terms would change only if GCVs are so close that numerical errors have an effect.

Note

Perhaps the most notable difference between mars and earth objects is that mars returns the MARS basis matrix in a field called "x" whereas earth returns "bx" with only the selected terms. Also, earth returns "dirs" rather than "factors", and in earth this matrix can have entries of value 2 for linear predictors.

For details of other differences between mars and earth objects, see the comments in the source code of mars.to.earth().

Weights

mda::mars normalises wp to (euclidean) length 1; earth normalises wp to length equal to the number of responses i.e. the number of columns in y. This change was made so an all 1s wp (or in fact any all constant wp) is equivalent to using no wp.

Note that the w argument is is actually ignored internally by mda::mars.

If the original call to mda::mars used the w or wp arguments, mars.to.earth will run update.earth to force consistency. This could modify the model, so a warning is issued.

See Also

earth, mars

Examples

if (require(mda)) {
    a <- mars(trees[,-3], trees[,3])
    a <- mars.to.earth(a)
    summary(a, digits = 2) # the standard earth functions can now be used

    # yields (note the reconstructed call):
    #    Call: earth(x=trees[, -3], y=trees[, 3])
    #
    #                    y
    #   (Intercept)  19.76
    #   h(Girth-12)   5.40
    #   h(12-Girth)  -2.56
    #   h(Height-76)  0.72
    #
    #   Selected 4 of 5 terms, and 2 of 2 predictors
    #   Importance: Girth, Height
    #   Number of terms at each degree of interaction: 1 3 (additive model)
    #   GCV 13    RSS 251    GRSq 0.95    RSq 0.97
}

[Package earth version 2.1-2 Index]