R: Q-Q Plot with Theoretical Distribution

B_04_qqmath {lattice} | R Documentation

## Q-Q Plot with Theoretical Distribution

### Description

Draw quantile-Quantile plots of a sample against a theoretical distribution, possibly conditioned on other variables.

### Usage

    qqmath(x, data, ...)

    ## S3 method for class 'formula'
    qqmath(x,
           data,
           allow.multiple = is.null(groups) || outer,
           outer = !is.null(groups),
           distribution = qnorm,
           f.value = NULL,
           auto.key = FALSE,
           aspect = "fill",
           panel = lattice.getOption("panel.qqmath"),
           prepanel = NULL,
           scales, strip, groups,
           xlab, xlim, ylab, ylim,
           drop.unused.levels = lattice.getOption("drop.unused.levels"),
           ...,
           lattice.options = NULL,
           default.scales = list(),
           default.prepanel = lattice.getOption("prepanel.default.qqmath"),
           subscripts,
           subset)
    ## S3 method for class 'numeric'
    qqmath(x, data = NULL, ylab, ...)

### Arguments

`x` | The object on which method dispatch is carried out. For the `"formula"` method, `x` should be a formula of the form `~ x | g1 * g2 * ...`, where `x` should be a numeric variable. For the `"numeric"` method, `x` should be a numeric vector.
`data` | For the `formula` method, an optional data source (usually a data frame) in which variables are to be evaluated (see `xyplot` for details). `data` should not be specified for the other methods, and is ignored with a warning if it is.
`distribution` | A quantile function that takes a vector of probabilities as argument and produces the corresponding quantiles from a theoretical distribution. Possible values are `qnorm`, `qunif`, etc. Distributions with other required arguments need to be provided as user-defined functions (see example with `qt`).
`f.value` | An optional numeric vector of probabilities, quantiles corresponding to which should be plotted. This can also be a function of a single integer (representing sample size) that returns such a numeric vector. A typical value for this argument is the function `ppoints`, which is also the S-PLUS default. If specified, the probabilities generated by this function is used for the plotted quantiles, through the `quantile` function for the sample, and the function specified as the `distribution` argument for the theoretical distribution. `f.value` defaults to `NULL`, which has the effect of using `ppoints` for the quantiles of the theoretical distribution, but the exact data values for the sample. This is similar to what happens for `qqnorm`, but different from the S-PLUS default of `f.value=ppoints`. For large `x`, this argument can be used to restrict the number of points plotted. See also the `tails.n` argument in `panel.qqmath`.
`panel` | A function, called once for each panel, that uses the packet (subset of panel variables) corresponding to the panel to create a display. The default panel function `panel.qqmath` is documented separately, and has arguments that can be used to customize its output in various ways. Such arguments can usually be directly supplied to the high-level function.
`allow.multiple, outer` | See `xyplot`.
`auto.key` | See `xyplot`.
`aspect` | See `xyplot`.
`prepanel` | See `xyplot`.
`scales` | See `xyplot`.
`strip` | See `xyplot`.
`groups` | See `xyplot`.
`xlab, ylab` | See `xyplot`.
`xlim, ylim` | See `xyplot`.
`drop.unused.levels` | See `xyplot`.
`lattice.options` | See `xyplot`.
`default.scales` | See `xyplot`.
`subscripts` | See `xyplot`.
`subset` | See `xyplot`.
`default.prepanel` | Fallback prepanel function. See `xyplot`.
`...` | Further arguments. See corresponding entry in `xyplot` for non-trivial details.

### Details

`qqmath` produces Q-Q plots of the given sample against a theoretical distribution. The default behaviour of `qqmath` is different from the corresponding S-PLUS function, but is similar to `qqnorm`. See the entry for `f.value` for specifics.

The implementation details are also different from S-PLUS. In particular, all the important calculations are done by the panel (and prepanel function) and not `qqmath` itself. In fact, both the arguments `distribution` and `f.value` are passed unchanged to the panel and prepanel function. This allows, among other things, display of grouped Q-Q plots, which are often useful. See the help page for `panel.qqmath` for further details.

This and all other high level Trellis functions have several arguments in common. These are extensively documented only in the help page for `xyplot`, which should be consulted to learn more detailed usage.

### Value

An object of class `"trellis"`. The `update` method can be used to update components of the object and the `print` method (usually called by default) will plot it on an appropriate plotting device.

### Author(s)

Deepayan Sarkar Deepayan.Sarkar@R-project.org

### See Also

`xyplot`, `panel.qqmath`, `panel.qqmathline`, `prepanel.qqmathline`, `Lattice`, `quantile`

### Examples

    qqmath(~ rnorm(100), distribution = function(p) qt(p, df = 10))
    qqmath(~ height | voice.part, aspect = "xy", data = singer,
           prepanel = prepanel.qqmathline,
           panel = function(x, ...) {
              panel.qqmathline(x, ...)
              panel.qqmath(x, ...)
           })
    vp.comb <-
        factor(sapply(strsplit(as.character(singer$voice.part), split = " "),
                      "[", 1),
               levels = c("Bass", "Tenor", "Alto", "Soprano"))
    vp.group <-
        factor(sapply(strsplit(as.character(singer$voice.part), split = " "),
                      "[", 2))
    qqmath(~ height | vp.comb, data = singer,
           groups = vp.group, auto.key = list(space = "right"),
           aspect = "xy",
           prepanel = prepanel.qqmathline,
           panel = function(x, ...) {
              panel.qqmathline(x, ...)
              panel.qqmath(x, ...)
           })

[Package lattice version 0.20-29 Index]