{smcl}
{* 30oct2003}{...}
{hline}
help for {hi:pexp}
{hline}

{title:Probability plot for data versus fitted exponential distribution}

{p 8 25 2}{cmd:pexp}{space 1}{it:varname} [{cmd:if} {it:exp}]
[{cmd:in} {it:range}] [{it:weight}] [{cmd:,} {cmdab:g:rid} 
{cmd:param(}{it:#}{cmd:)} {cmd:show(}{it:condition}{cmd:)} 
{cmdab:gen:erate(}{it:newvar1 newvar2}{cmd:)} 
{it:graph_options} ]

{p 4 4 2}
where {it:graph_options} are

{p 8 8 2}
{cmd:rlopts(}{it:cline_options}{cmd:)}
{cmd:plot(}{it:plot}{cmd:)}
{it:scatter_options}
{it:twoway_options}

{p 4 4 2}{cmd:fweight}s, {cmd:aweight}s and {cmd:iweight}s 
are allowed; see help {help weights}. 

{title:Description}

{p 4 4 2}
{cmd:pexp} produces a probability plot for {it:varname} compared with a
one-parameter exponential distribution with probability density function 
for variable {it:x} of (1/{it:m}) exp(-{it:x}/{it:m}). The parameter {it:m}
is by default calculated as the mean of the data specified. 


{title:Remarks} 

{p 4 4 2}In the majority of cases, {cmd: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 {cmd:param()}
option. In all cases it is important not only to specify any {cmd:if} or
{cmd:in} restrictions, but also to specify relevant weights, which will (unless
constant) affect the configuration of the plot. 


{title:Options}

{p 4 8 2}{cmd:grid} is equivalent to {cmd:yla(0(.25)1, grid) xla(0(.25)1, grid)}.

{p 4 8 2}{cmd:param()} may be used to supply a parameter value (namely, the 
mean) directly for use in estimation of fitted quantiles. 

{p 4 8 2}{cmd: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 {cmd:if} 
and {cmd:in} should not be used for this purpose. 

{p 4 8 2}{cmd:generate()} specifies the names of two new variables, 
the first to hold exponential probabilities and the second to hold 
empirical probabilities. 

{p 4 8 2}{it:graph_options} are
{cmd:rlopts(}{it:cline_options}{cmd:)},
{cmd:plot(}{it:plot}{cmd:)},
{it:scatter_options}, and
{it:twoway_options}

{p 4 8 2}
{cmd:rlopts(}{it:cline_options}{cmd:)} affect the rendition of the reference
line; see help {help cline_options}.

{p 4 8 2}
{cmd:plot(}{it:plot}{cmd:)} provides a way to add other plots to the generated
graph; see help {help plot_option}.

{p 4 8 2}
{it:scatter_options} affect the rendition of the plotted points; see help
{help scatter}.

{p 4 8 2}
{it:twoway_options} are any of the options documented in help
{help twoway_options} excluding {cmd:by()}.  These include options for titling
the graph (see help {help title_options}) and options for saving the graph to
disk (see help {help saving_option}).


{title:Examples}

{p 4 8 2}{cmd:. pexp spacing}

{p 4 8 2}{cmd:. pexp spacing, param(20)}

{p 4 8 2}{cmd:. pexp spacing, gen(spacing_exp spacing_emp)} 


{title:Author}

{p 4 4 2}Nicholas J. Cox, University of Durham, U.K.{break} 
n.j.cox@durham.ac.uk


{title:Also see}

{p 4 13 2}
Online:  help for {help qexp} (if installed), {help graph}, {help diagplots}