Attribute VB_Name = "TDIST"

' Copyright (c) 1997 C. David Eagle Jr.

DefInt I-N
DefDbl A-H, O-Z

Static Sub TDIST(N, X, PROB)
   
    ' T-distribution subroutine

    ' Input

    '  N = degrees of freedom; N >= 1
    '  X = function argument; X > 0

    ' Output

    '  PROB = probability; 0 < PROB <= 1

    A = 0.36338023

    W = Atn(X / Sqr(N))
    S = Sin(W)
    C = Cos(W)
    L = N - 2 * Int(N / 2)

    If (L = 0) Then GoTo L10

    T1 = W

    If (N = 1) Then GoTo L30

    T2 = S * C
    T1 = T1 + T2

    If (N = 3) Then GoTo L30

    J1 = 0
    J2 = 1
    K2 = (N - 3) / 2

    GoTo L20

L10:
    T1 = S

    If (N = 2) Then GoTo L40

    T2 = S
    J1 = -1
    J2 = 0
    K2 = (N - 2) / 2

L20:
    For I = 1 To K2
        J1 = J1 + 2
        J2 = J2 + 2
        T2 = T2 * C * C * J1 / J2
        T1 = T1 + T2
    Next I

L30:
    T1 = T1 * (1# - A * L)

L40:
    PROB = 0.5 * (1# + T1)

End Sub