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help for pweibull
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Probability plot for data versus fitted Weibull distribution

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

where graph_options are

twoway_options

fweights and aweights are allowed; see help weights.

Description

pweibull produces a probability plot for varname compared with a
two-parameter Weibull distribution with probability density function for
variable x >= 0, scale parameter b > 0 and shape parameter c > 0 of (c/b)
(x/b)^(c - 1) exp(-(x/b)^c).  By default, maximum likelihood estimation
is carried out by weibullfit.

Remarks

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

param() may be used to supply parameter values (namely, scale and shape
parameters) directly for use in estimation of fitted quantiles.  b
and c 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 names of two new variables, the first to hold
Weibull probabilities and the second to hold empirical probabilities.

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

. pweibull spacing

. pweibull spacing, param(20 3)

. pweibull spacing, gen(spacing_wei spacing_emp)

Author

Nicholas J. Cox, Durham University, 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. 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

Online:  help for qweibull (if installed), weibullfit, graph

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