Dataplot Vol 2 Vol 1

# TRAPPF

Name:
TRAPPF (LET)
Type:
Library Function
Purpose:
Compute the trapezoidal percent point function.
Description:
The trapezoidal distribution has the following cumulative distribution function:

The algorithm for the trapezoid percent point function is to compute the cumulative distribution function at the points a, b, c, and d. This is used to determine which of the four intervals contains the percent point value. Then the appropriate equation above is inverted to find the value of the percent point function.

Syntax:
LET <y> = TRAPPF(<p>,<a>,<b>,<c> <d>)
<SUBSET/EXCEPT/FOR qualification>
where <p> is a variable, number, or parameter containing values in the interval (0,1);
<a> is a number, parameter, or variable that specifies the first shape parameter;
<b> is a number, parameter, or variable that specifies the second shape parameter;
<c> is a number, parameter, or variable that specifies the third shape parameter;
<d> is a number, parameter, or variable that specifies the fourth shape parameter;
<y> is a variable or a parameter (depending on what <p> is) where the computed pdf value is stored;
and where the <SUBSET/EXCEPT/FOR qualification> is optional.
Examples:
LET A = TRAPPF(0.95,0,0.25,0.75,1)
LET A = TRAPPF(0.05,A,B,C,D)
Default:
None
Synonyms:
None
Related Commands:
 TRACDF = Compute the trapezoidal cumulative distribution function. TRAPDF = Compute the trapezoidal probability density function. TRIPDF = Compute the triangular probability density function. UNIPDF = Compute the uniform probability density function. SEMPDF = Compute the semi-circular probability density function. NORPDF = Compute the normal probability density function.
Reference:
"Generalized Trapezoidal Distributions", van Dorp and Kotz, Metrika, Vol. 58, Issue 1, July, 2003.
Applications:
Distributional Modeling
Implementation Date:
2004/1
Program:
```
TITLE TRAPEZOIDAL DISTRIBUTION
LET A = 0
LET B = 1
LET C = 5
LET D = 10
PLOT TRAPPF(P,A,B,C,D) FOR P = 0  0.01  1
```

Date created: 2/3/2004
Last updated: 2/3/2004