OOps! Forgot to remove Worksheetfunction.

Option Explicit
Const SqrPi As Double = 1.77245385090552

Sub TestIt()
Debug.Print InverseErf(0.1)
Debug.Print InverseErf(0.5)
Debug.Print InverseErf(0.9)
End Sub

Function InverseErf(n)
If n < 0 Or n = 1 Then Exit Function

Dim g As Double
Dim j As Long
g = 0
For j = 1 To 15
g = g + (Exp(g * g) * SqrPi * (n - Erf(g))) / 2
Next j
InverseErf = g
End Function

Function Erf(z)
Dim k As Long
Dim F As Double '(F)actorial
Dim n As Double
Dim d As Double
Dim Ans As Double

If z = 0 Then Exit Function
F = 1
For k = 0 To 25
n = (-1) ^ k * z ^ (2 * k + 1)
d = F * (2 * k + 1)
Ans = Ans + n / d
F = F * (k + 1)
Next k
Erf = Ans * 2 / SqrPi
End Function

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HTH :)
Dana DeLouis