The folded t percent point function is computed using the t percent point as follows:
with tppf denoting the t percent point function and denoting the degrees of freedom parameter.
The folded t distribution provides an alternative to the half-normal or half-Cauchy in distributional modeling applications. A folded t with 1 degree of freedom is equivalent to a half-Cauchy and the folded t approximates the half-normal as the degrees of freedom gets large (in practice, the approximation is quite good for degrees of freedom > 30). Thus the folded t allows you to model with tails that can vary from half-normal to half-Cauchy in behavior.
where <p> is a variable, a number or a parameter in the range (0,1];
<y> is a variable or a parameter (depending on what <p> is) where the computed folded t ppf value is stored;
<nu> is a positive number or parameter that specifies the degrees of freedom;
and where the <SUBSET/EXCEPT/FOR qualification> is optional.
LET Y2 = FTPPF(P1,10)
PLOT FTPPF(P,3) FOR P = 0 0.01 0.95
MULTIPLOT CORNER COORDINATES 0 0 100 100 MULTIPLOT SCALE FACTOR 2 MULTIPLOT 2 2 TITLE AUTOMATIC PLOT FTPPF(P,1) FOR X = 0 0.01 0.99 PLOT FTPPF(P,5) FOR X = 0 0.01 0.99 PLOT FTPPF(P,10) FOR X = 0 0.01 0.99 PLOT FTPPF(P,30) FOR X = 0 0.01 0.99 END OF MULTIPLOT
Date created: 2/3/2004