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help for qbeta 
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Quantile-quantile plot for data versus fitted beta distribution

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

    where graph_options are

        rlopts(cline_options) plot(plot) scatter_options twoway_options

    fweights and aweights are allowed; see help weights.


Description

    qbeta plots the quantiles of varname against the quantiles of a
    two-parameter beta distribution with probability density function for
    variable 1 > x > 0, shape parameters a > 0 and b > 0 of [1 / B(a, b) ] x
    ^(a - 1) (1 -x)^ (b - 1).  Here B(,) is the beta function.  By default,
    maximum likelihood estimation is carried out, using betafit, which should
    be installed separately.


Remarks 

    In the majority of cases, qbeta will be used to fit an beta distribution
    on the fly and to assess that fit. In some cases, it may be of interest
    to compare data with a beta 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

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

    param() may be used to supply parameter values 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
        beta quantiles.

    graph_options are rlopts(cline_options), plot(plot), 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.

    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

    . gen unif = uniform()
    . qbeta unif, param(1 1)


Author

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


References

    Evans, M., Hastings, N. and Peacock, B. 2000. Statistical distributions.
    New York: John Wiley.

    Johnson, N.L., Kotz, S. and Balakrishnan, N. 1995.  Continuous univariate
    distributions: Volume 2. New York: John Wiley.


Also see