x2分布分位数m.bas

<VB数理统计实用算法>书中的算法源程序

Attribute VB_Name = "modMethod"
Option Explicit
'计算卡方分布的分位数
'n：自由度
'Q：上侧概率
'xx：分位数
Public Sub PCX2(n As Integer, Q As Double, xx As Double)
    Dim I As Integer, x As Double, p As Double, W As Double
    Dim x0 As Double, pp As Double, d As Double
    If n = 1 Then
        PNorm Q / 2, x: xx = x * x
        Exit Sub
    End If
    If n = 2 Then
        xx = -2 * Log(Q)
        Exit Sub
    End If
    p = 1 - Q: PNorm Q, x: W = 2 / (9 * n)
    x0 = n * (1 - W + x * Sqr(W)) ^ 3
    For I = 1 To 30
        CX2 n, x0, pp, d
        If d = 0 Then
            xx = x0
            Exit Sub
        End If
        xx = x0 - (pp - p) / d
        If Abs(xx - x0) < 10 - 6 * Abs(xx) Then Exit Sub Else x0 = xx
    Next I
End Sub

'求正态分布的分位数
'Q：上侧概率
'x：分位数
Public Sub PNorm(Q, x)
    Dim p As Double, y As Double, z As Double
    Dim b0 As Double, b1 As Double, b2 As Double
    Dim b3 As Double, b4 As Double, b5 As Double
    Dim b6 As Double, b7 As Double, b8 As Double
    Dim b9 As Double, b10 As Double, b As Double
    b0 = 1.570796288
    b1 = 0.03706987906
    b2 = -0.0008364353589
    b3 = -0.0002250947176
    b4 = 0.000006841218299
    b5 = 0.000005824238515
    b6 = -0.00000104527497
    b7 = 8.360937017E-08
    b8 = -3.231081277E-09
    b9 = 3.657763036E-11
    b10 = 6.936233982E-13
    If Q = 0.5 Then
        x = 0: GoTo PN01
    End If
    If Q > 0.5 Then p = 1 - Q Else p = Q
    y = -Log(4 * p * (1 - p))
    b = y * (b9 + y * b10)
    b = y * (b8 + b)
    b = y * (b7 + b)
    b = y * (b6 + b)
    b = y * (b5 + b)
    b = y * (b4 + b)
    b = y * (b3 + b)
    b = y * (b2 + b)
    b = y * (b1 + b)
    z = y * (b0 + b)
    x = Sqr(z)
    If Q > 0.5 Then x = -x
PN01:
End Sub

'计算卡方分布函数和概率密度
'n：自由度
'x2：卡方值
'F：下侧概率
'd：概率密度
Public Sub CX2(n As Integer, X2 As Double, F As Double, d As Double)
    Dim PIS As Double, x As Double, CHS As Double, u As Double
    Dim IAI As Integer, pp As Double, N2 As Integer, I As Integer
    Const PI As Double = 3.14159265359
    If X2 = 0 Then
        F = 0: d = 0: Exit Sub
    End If
    PIS = Sqr(PI)
    x = X2 / 2
    CHS = Sqr(X2)
    If (n \ 2) * 2 = n Then                 'n为偶数
        u = x * Exp(-x)
        F = 1 - Exp(-x)
        IAI = 2
    Else                                    'n为奇数
        u = Sqr(x) * Exp(-x) / PIS
        Norm CHS, pp                        '调用正态分布函数计算过程
        F = 2 * (pp - 0.5)
        IAI = 1
    End If
    If IAI = n Then GoTo LL1 Else N2 = n - 2
    For I = IAI To N2 Step 2
        F = F - 2 * u / I
        u = X2 * u / I
    Next I
LL1:
    d = u / X2
End Sub

'计算正态分布函数
'x：正态偏离点
'F：下侧概率
Public Sub Norm(x, F)
    Dim y As Double, ER As Double, Q As Double
    Dim A
    Const a1 As Double = 0.0705230784
    Const a2 As Double = 0.0422820123
    Const a3 As Double = 0.0092705272
    Const a4 As Double = 0.0001520143
    Const a5 As Double = 0.0002765672
    Const a6 As Double = 0.0000430638
    y = 0.707106781187 * Abs(x)
    A = a4 + y * (a5 + y * a6)
    A = a3 + y * A
    A = a2 + y * A
    A = a1 + y * A
    ER = 1 - (1 + y * A) ^ (-16)
    Q = 0.5 * ER
    If x < 0 Then F = 0.5 - Q Else F = 0.5 + Q
End Sub