Quantile-Quantile Plots of Two Samples

Quantile-Quantile plots for comparing two Distributions

Usage

qq(x, data, ...)

# S3 method for class 'formula'
qq(x, data, aspect = "fill", 
   panel = lattice.getOption("panel.qq"),
   prepanel, scales, strip, 
   groups, xlab, xlim, ylab, ylim, f.value = NULL, 
   drop.unused.levels = lattice.getOption("drop.unused.levels"),
   ...,
   lattice.options = NULL,
   qtype = 7,
   default.scales = list(),
   default.prepanel = lattice.getOption("prepanel.default.qq"),
   subscripts,
   subset)

# S3 method for class 'data.frame'
qq(x, data = NULL, formula = data, ...)

Arguments

x

The object on which method dispatch is carried out.

For the "formula" method, x should be a formula of the form y ~ x | g1 * g2 * ..., where x should be a numeric variable, and y a factor, shingle, character, or numeric variable, with the restriction that there must be exactly two levels of y, which divide the values of x into two groups. Quantiles for these groups will be plotted against each other along the two axes.

data

For the formula method, an optional data source (usually a data frame) in which variables are to be evaluated (see xyplot for details).

formula

The formula to be used for the "data.frame" method. See documentation for argument x for details.

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.

f.value defaults to NULL, which is equivalent to

      f.value = function(n) ppoints(n, a = 1)
    

This has the effect of including the minimum and maximum data values in the computed quantiles. This is similar to what happens for qqplot but different from the default behaviour of qq in S-PLUS.

For large x, this argument can be used to restrict the number of quantiles plotted.

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.qq 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.

qtype

The type argument for quantile.

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.

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

Deepayan Sarkar Deepayan.Sarkar@R-project.org

Details

qq produces Q-Q plots of two samples. The default behaviour of qq is different from the corresponding S-PLUS function. See the entry for f.value for specifics.

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.

See also

xyplot, panel.qq, qqmath, Lattice

Examples

qq(voice.part ~ height, aspect = 1, data = singer,
   subset = (voice.part == "Bass 2" | voice.part == "Tenor 1"))