sm.ps {VGAM} | R Documentation

## Defining Penalized Spline Smooths in VGAM Formulas

### Description

This function represents a P-spline smooth term in a `vgam` formula and confers automatic smoothing parameter selection.

### Usage

    sm.ps(x, ..., ps.int = NULL, spar = -1, degree = 3, p.order = 2,
          ridge.adj = 1e-5, spillover = 0.01, maxspar = 1e12,
          outer.ok = FALSE, mux = NULL, fixspar = FALSE)

### Arguments

`x, ...` | See `sm.os`.

`ps.int`

the number of equally-spaced B-spline intervals. Note that the number of knots is equal to `ps.int + 2*degree + 1`. The default, signified by `NULL`, means that the maximum of the value 7 and `degree` is chosen. This usually means 6 interior knots for big data sets. However, if this is too high compared to the length of `x`, then some adjustment is made. In the case where `mux` is assigned a numerical value (suggestions: some value between 1 and 2) then `ceiling(mux * log(length(unique(x.index))))` is used, where `x.index` is the combined data. No matter what, the above is not guaranteed to work on every data set. This argument may change in the future. See also argument `mux`.

`spar, maxspar` | See `sm.os`.
`mux` | numeric. If given, then this argument multiplies `log(length(unique(x)))` to obtain `ps.int`. If `ps.int` is given then this argument is ignored.
`degree` | degree of B-spline basis. Usually this will be 2 or 3; and the values 1 or 4 might possibly be used.
`p.order` | order of difference penalty (0 is the ridge penalty).
`ridge.adj, spillover` | See `sm.os`.
`outer.ok, fixspar` | See `sm.os`.

### Details

This function can be used by `vgam` to allow automatic smoothing parameter selection based on P-splines and minimizing an UBRE quantity.

This function should only be used with `vgam` and is an alternative to `sm.os`; see that function for some details that also apply here.

### Value

A matrix with attributes that are (only) used by `vgam`. The number of rows of the matrix is `length(x)` and the number of columns is `ps.int + degree - 1`. The latter is because the function is centred.

### Warning

See `sm.os`.

### Note

This function is currently under development and may change in the future. In particular, the default for `ps.int` is subject to change.

### Author(s)

B. D. Marx wrote the original function. Subsequent edits were made by T. W. Yee and C. Somchit.

### References

Eilers, P. H. C. and Marx, B. D. (1996). Flexible smoothing with B-splines and penalties (with comments and rejoinder). Statistical Science, 11(2): 89–121.

### See Also

`sm.os`, `s`, `vgam`, `smartpred`, `is.smart`, `summarypvgam`, `splineDesign`, `bs`, `magic`.

### Examples

    sm.ps(runif(20))
    sm.ps(runif(20), ps.int = 5)

    ## Not run:
    data("TravelMode", package = "AER")  # Need to install "AER" first
    air.df <- subset(TravelMode, mode == "air")  # Form 4 smaller data frames
    bus.df <- subset(TravelMode, mode == "bus")
    trn.df <- subset(TravelMode, mode == "train")
    car.df <- subset(TravelMode, mode == "car")
    TravelMode2 <- data.frame(income     = air.df$income,
                              wait.air   = air.df$wait  - car.df$wait,
                              wait.trn   = trn.df$wait  - car.df$wait,
                              wait.bus   = bus.df$wait  - car.df$wait,
                              gcost.air  = air.df$gcost - car.df$gcost,
                              gcost.trn  = trn.df$gcost - car.df$gcost,
                              gcost.bus  = bus.df$gcost - car.df$gcost,
                              wait       = air.df$wait)  # Value is unimportant
    TravelMode2$mode <- subset(TravelMode, choice == "yes")$mode  # The response
    TravelMode2 <- transform(TravelMode2, incom.air = income, incom.trn = 0,
                                          incom.bus = 0)
    set.seed(1)
    TravelMode2 <- transform(TravelMode2,
                             junkx2 = runif(nrow(TravelMode2)))

    tfit2 <-
      vgam(mode ~ sm.ps(gcost.air, gcost.trn, gcost.bus) + ns(junkx2, 4) +
                  sm.ps(incom.air, incom.trn, incom.bus) + wait ,
           crit = "coef",
           multinomial(parallel = FALSE ~ 1), data = TravelMode2,
           xij = list(sm.ps(gcost.air, gcost.trn, gcost.bus) ~
                      sm.ps(gcost.air, gcost.trn, gcost.bus) +
                      sm.ps(gcost.trn, gcost.bus, gcost.air) +
                      sm.ps(gcost.bus, gcost.air, gcost.trn),
                      sm.ps(incom.air, incom.trn, incom.bus) ~
                      sm.ps(incom.air, incom.trn, incom.bus) +
                      sm.ps(incom.trn, incom.bus, incom.air) +
                      sm.ps(incom.bus, incom.air, incom.trn),
                      wait   ~  wait.air +  wait.trn +  wait.bus),
           form2 = ~  sm.ps(gcost.air, gcost.trn, gcost.bus) +
                      sm.ps(gcost.trn, gcost.bus, gcost.air) +
                      sm.ps(gcost.bus, gcost.air, gcost.trn) +
                      wait +
                      sm.ps(incom.air, incom.trn, incom.bus) +
                      sm.ps(incom.trn, incom.bus, incom.air) +
                      sm.ps(incom.bus, incom.air, incom.trn) +
                      junkx2 + ns(junkx2, 4) +
                      incom.air + incom.trn + incom.bus +
                      gcost.air + gcost.trn + gcost.bus +
                      wait.air +  wait.trn +  wait.bus)
    par(mfrow = c(2, 2))
    plot(tfit2, se = TRUE, lcol = "orange", scol = "blue", ylim = c(-4, 4))
    summary(tfit2)

    ## End(Not run)

[Package VGAM version 1.1-1 Index]