TNPR
This function evaluates the noncentral Student's t probability density function.
Function Return Value
TNPR — Function value, the value of the probability density function. (Output)
Required Arguments
T — Argument for which the noncentral Student's t probability density function is to be evaluated. (Input)
DF — Number of degrees of freedom of the noncentral Student's t distribution. (Input)
DF must be positive.
DELTA — Noncentrality parameter. (Input)
FORTRAN 90 Interface
Generic: TNPR (T, DF, DELTA)
Specific: The specific interface names are S_TNPR and D_TNPR.
Description
The noncentral Student's t distribution is a generalization of the Student's t distribution.
If w is a normally distributed random variable with unit variance and mean δ and u is a chi‑square random variable with ν degrees of freedom that is statistically independent of w, then
is a noncentral t‑distributed random variable with ν degrees of freedom and noncentrality parameter δ, that is, with ν = DF, and δ = DELTA. The probability density function for the noncentral t‑distribution is:
where
and t = T.
For δ = 0, the PDF reduces to the (central) Student’s t PDF:
and, for t = 0, the PDF becomes:
Example
This example calculates the noncentral Student’s t PDF for a distribution with 2 degrees of freedom and noncentrality parameter δ = 10.
 
      USE TNPR_INT
      USE UMACH_INT
      IMPLICIT NONE
     
      INTEGER  :: NOUT, I
      REAL     :: X(6)=(/ -.5, 1.5, 3.5, 7.5, 51.5, 99.5 /)
      REAL     :: DF, DELTA, PDFV
 
      CALL UMACH (2, NOUT)
      DF = 2.0
      DELTA = 10.0
 
      WRITE (NOUT,'("DF: ", F4.0, " DELTA: ", F4.0 // &
             " X PDF(X)")') DF, DELTA
 
      DO I = 1, 6
         PDFV = TNPR(X(I), DF, DELTA)
         WRITE (NOUT,'(1X, F4.1, 2X, E12.5)') X(I), PDFV
      END DO
      END
Output
 
DF:   2.  DELTA:  10.
 
   X        PDF(X)
 -0.5   0.16399E-23
  1.5   0.74417E-09
  3.5   0.28972E-02
  7.5   0.78853E-01
 51.5   0.14215E-02
 99.5   0.20290E-03