help cquantile
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Title

    cquantile -- generate corresponding quantiles


Syntax

        cquantile varname1 varname2 [if] [in] , generate(newvar1 newvar2)

        cquantile varname [if] [in] , by(groupvar) generate(newvar1 newvar2)


Description

    cquantile generates corresponding quantiles, namely, those quantiles that
    would be shown on a quantile-quantile plot, as in qqplot. Given either
    two numeric variables, or one numeric variable and one grouping variable
    defining two groups, two new variables are generated containing the
    quantiles.  One of those new variables contains values identical to those
    in one of the existing variables or groups, and the other contains
    corresponding quantiles.  The two new variables are aligned lowest to
    lowest, ..., highest to highest.  There is thus some redundancy
    introduced, but having both variables handy is intended to be a
    convenience, especially for further graphics. For example,
    quantile-quantile plots are often inspected for additive or
    multiplicative structure. A direct way to check for either would be to
    plot difference of quantiles versus their mean or ratio of quantiles
    versus their geometric mean.


Options

    generate() specifies two new variable names to hold the quantiles.

    by() specifies a group variable to specify precisely two groups of
        varname, after any if or in restrictions have been noted. by() may be
        only be specified with a single variable name.


Examples

    . cquantile length width, gen(Qlength Qwidth)
    . gen diff = Qlength - Qwidth
    . gen mean = (Qlength + Qwidth) / 2
    . scatter diff mean, yti(length - width) xti((length + width)/2)

    . cquantile mpg, by(foreign) generate(mpg0 mpg1)
    . qqplot mpg?


Author

    Nicholas J. Cox, Durham University
    n.j.cox@durham.ac.uk


Also see

    Manual:  [R] diagnostic plots

    Online:  qqplot, separate, fixsort (if installed), pairplot (if
             installed)