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

{title:Quantile-quantile plot for data versus fitted exponential distribution}

{p 8 25 2}{cmd:qexp}{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:newvar}{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:qexp} plots the quantiles of {it:varname} against the quantiles of 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:qexp} 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} adds grid lines at the .05, .10, .25, .50, .75, .90, and .95
quantiles. 

{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 name of a new variable to hold values 
of fitted exponential quantiles. 

{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:. qexp spacing}

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

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


{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 pexp} (if installed), {help graph}, {help diagplots}