## Model Definition for Exponential Distribution

```/*--------------------------------------------------------------

SAS Sample Library

Name: svrtexp.sas
Description: Example Program from SAS/ETS User's Guide,
The SEVERITY Procedure
Title: Model Definition for Exponential Distribution
Product: SAS/ETS Software
Keys: fitting continuous distributions
PROC: SEVERITY
Notes: If you run this sample program without any modification, then
the Sasuser.Svrtdist library contains functions and subroutines
that are identical to those in the Sashelp.Svrtdist library.

--------------------------------------------------------------*/
proc fcmp outlib=sasuser.svrtdist.models;
function EXP_DESCRIPTION() \$32;
length model \$32;
model = "Exponential Distribution";
return(model);
endsub;

function EXP_PARMCOUNT();
return(1);
endsub;

function EXP_LOGPDF(x,Theta);
/* Theta : Scale */
if (x < 0) then
return (-180.218266); /* constant('LOGSMALL') */
else
return (-x/Theta - log(Theta));
endsub;
function EXP_PDF(x,Theta);
/* Theta : Scale */
if (x < 0) then
return (0);
else
return (exp(-x/Theta)/Theta);
endsub;

function EXP_LOGCDF(x,Theta);
/* Theta : Scale */
if (x < 0) then
return (-180.218266); /* constant('LOGSMALL') */
else do;
c = exp(-x/Theta);
if (c < 1) then
return (log1px(-c));
else
return (-180.218266); /* constant('LOGSMALL') */
end;
endsub;
function EXP_CDF(x,Theta);
/* Theta : Scale */
if (x < 0) then
return (0);
else
return (1.0 - exp(-x/Theta));
endsub;

function EXP_LOGSDF(x,Theta);
/* Theta : Scale */
if (x < 0) then
return (0);
else
return (-x/Theta);
endsub;
function EXP_SDF(x,Theta);
/* Theta : Scale */
if (x < 0) then
return (1);
else
return (exp(-x/Theta));
endsub;

subroutine EXP_PARMINIT(dim, x[*], nx[*], F[*], Ftype, Theta);
outargs Theta;

/* Compute estimate using method of moments */
Theta = 0;
n = 0;
do i=1 to dim;
Theta = Theta + nx[i] * x[i];
n = n + nx[i];
end;
if (n = 0) then
Theta = .;
else
Theta = Theta / n;
endsub;

function EXP_QUANTILE(p,Theta);
/* Theta : Scale */
if (p < 0 or p > 1) then
return (.);
else
return (quantile('EXPO',p,Theta));
endsub;

function EXP_MEAN(x, Theta);
return (Theta);
endsub;
quit;

```