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.
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.
LET A = TRAPPF(0.05,A,B,C,D)
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