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help for rhetplot
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Plot checking for residual heteroscedasticity after model fit

        rhetplot , by(byvarlist) [residualtype variance lowess_options
                 plot(plot) ]

        rhetplot varname [, at(numlist) | group(#)] [residualtype variance
                 lowess_options plot(plot) ]

        rhetplot [, at(numlist) | group(#)] residualtype variance
                 lowess_options plot(plot) ]


Description

    rhetplot is for use after regress and similar commands; see help on the
    command of interest.

    With the first syntax, rhetplot graphs standard deviations (optionally
    variances) of residuals for the distinct groups formed by combinations of
    byvarlist.  If byvarlist is a single numeric variable, it is used for the
    x axis; otherwise byvarlist is used as the argument to egen, group()
    label to define a grouped variable. See egen.

    With the second syntax, rhetplot graphs standard deviations (optionally
    variances) of residuals against means of groups of varname.  Groups are
    defined by the argument of either at() or cut(), used as the
    corresponding option for egen, cut(). See egen.

    With the third syntax, rhetplot graphs standard deviations (optionally
    variances) of residuals against means of groups of fitted values from the
    model.  Groups are defined by the argument of either at() or cut(), used
    as the corresponding option for egen, cut().  See egen.

    The residuals are, by default, those calculated by predict, residuals or
    (if the previous estimation command was glm) by predict, response.

    The graph is produced by lowess. The `smooth' curve shown (unless the
    number of groups specified is very small) is best regarded as an informal
    indication of the general pattern of variability of residuals.

    The graph is restricted to the estimation sample.


Options

    by(byvarlist), required with the first syntax, specifies that the plot
        should be of variability of residuals among the distinct groups
        defined by combinations of byvarlist.

    at(numlist) or group(#), required with the second and third syntaxes,
        specifies how groups of either a specified variable (second syntax)
        or fitted values (third syntax) are to be determined.  at() and
        group() are options of egen, cut(): see help on egen.  In brief, at()
        specifies lower limits of user-defined classes and group() specifies
        the number of quantile-based groups with (approximately) equal
        frequency to be used. (With at(), supply one high value beyond the
        maximum of the data for best results.)

    residualtype specifies a type of residual other than the default.  The
        following types are currently supported: anscombe, deviance,
        likelihood, pearson, residuals, response, rstandard, rstudent, score,
        working.

    variance specifies plotting of variances rather than standard deviations.
        Occasionally, especially with counted variables, the natural question
        may be how far variance is constant or a function of mean fitted
        values.

    lowess_options are any of the options allowed with lowess.

    plot(plot) provides a way to add other plots to the generated graph; see 
        plot_option.


Examples

    . webuse systolic
    . anova systolic drug disease
    . rhetplot, by(drug disease) xla(1/12, valuelabel)

    . sysuse auto
    . glm mpg weight, link(power -1)
    . rhetplot weight, g(7)

    . reg turn length
    . regplot
    . rhetplot, g(7)
    . rhetplot length, g(7)

    . insheet using http://www.kgs.ku.edu/Mathgeo/Books/Stat/ASCII/OCS.TXT,
        clear
    . label data "petroleum reservoirs, Outer Continental Shelf, Texas and
        Louisiana"
    . label var mmboe "ultimate production, million barrels oil equivalent"
    . label var area "area of closure, acres"
    . regress mmboe area
    . rhetplot area, g(7)
    . regress log10mmboe log10area
    . rhetplot log10area, g(7)


Author

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


Also see

    On-line:  help for predict, modeldiag