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help for qgamma 
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Quantile-quantile plot for data versus fitted gamma distribution

        qgamma varname [if exp] [in range] [weight] [, alternative grid
                         param(# #) show(condition) generate(newvar)
                         graph_options ]

    where graph_options are

        rlopts(cline_options) plot(plot) addplot(plot) scatter_options
        twoway_options

    fweights and aweights are allowed; see help weights.


Description

    qgamma plots the quantiles of varname against the quantiles of a
    two-parameter gamma distribution with probability density function for
    variable x >= 0, shape parameter a > 0 and scale parameter b > 0 of [1 /
    (b^a gamma(a))] x^(a - 1) exp(-x / b).  See also the alternative option.
    By default, maximum likelihood estimation is carried out, using gammafit,
    which should be installed separately.


Remarks 

    In the majority of cases, qgamma will be used to fit an gamma
    distribution on the fly and to assess that fit. In some cases, it may be
    of interest to compare data with a gamma distribution with known or
    hypothesised parameters, which may be specified using the param() option.
    In all cases it is important not only to specify any if or in
    restrictions, but also to specify relevant weights, which will (unless
    constant) affect the configuration of the plot.


Options

    alternative specifies use of an alternative parameterisation [(b^a /
        gamma(a))] x^(a - 1) exp(-bx).

    grid adds grid lines at the .05, .10, .25, .50, .75, .90, and .95
        quantiles.

    param() may be used to supply parameter values (namely, shape and scale
        parameters) directly for use in estimation of fitted quantiles.  a
        and b should be provided as separate values in precisely that order.

    show() may be used to specify that you wish to restrict the graph
        according to some condition, say looking at one tail of the
        distribution only. Note that if and in should not be used for this
        purpose.

    generate() specifies the name of a new variable to hold values of fitted
        gamma quantiles.

    graph_options are rlopts(cline_options), plot(plot) (Stata 8 only),
        addplot(plot) (Stata 9 up), scatter_options , and twoway_options.

    rlopts(cline_options) affect the rendition of the reference line; see
        help cline_options.

    plot(plot) provides a way to add other plots to the generated graph; see
        help plot_option (Stata 8 only).

    addplot(plot) provides a way to add other plots to the generated graph;
        see help addplot_option (Stata 9 up).

    scatter_options affect the rendition of the plotted points; see help 
        scatter.

    twoway_options are any of the options documented in help twoway_options
        excluding by().  These include options for titling the graph (see
        help title_options) and options for saving the graph to disk (see
        help saving_option).


Examples

    . qgamma spacing

    . qgamma spacing, param(20 3)

    . qgamma spacing, gen(spacing_g)


Author

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


References

    Forbes, C., Evans, M., Hastings, N. and Peacock, B. 2011.  Statistical
    distributions.  New York: John Wiley.

    Johnson, N.L., Kotz, S. and Balakrishnan, N. 1994.  Continuous univariate
    distributions: Volume 1. New York: John Wiley.

    Kleiber, C. and Kotz, S. 2003.  Statistical size distributions in
    economics and actuarial sciences.  Hoboken, NJ: John Wiley.


Also see