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help for pexp
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Probability plot for data versus fitted exponential distribution

        pexp varname [if exp] [in range] [weight] [, grid param(#)
                         show(condition) generate(newvar1 newvar2)
                         graph_options ]

    where graph_options are

        rlopts(cline_options) plot(plot) scatter_options twoway_options

    fweights, aweights and iweights are allowed; see help weights.

Description

    pexp produces a probability plot for varname compared with a
    one-parameter exponential distribution with probability density function
    for variable x of (1/m) exp(-x/m). The parameter m is by default
    calculated as the mean of the data specified.


Remarks 

    In the majority of cases, pexp will be used to fit an exponential
    distribution on the fly and to assess that fit. In some cases, it may be
    of interest to compare data with an exponential distribution with a known
    or hypothesised mean, 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 is equivalent to yla(0(.25)1, grid) xla(0(.25)1, grid).

    param() may be used to supply a parameter value (namely, the mean)
        directly for use in estimation of fitted quantiles.

    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 names of two new variables, the first to hold
        exponential probabilities and the second to hold empirical
        probabilities.

    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

    . pexp spacing

    . pexp spacing, param(20)

    . pexp spacing, gen(spacing_exp spacing_emp)


Author

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


Also see