I have adapted an algorithm for calculating the standard deviation of
levels (20Log(x)) in an Excel function. The function seems to work
correctly but when I reopen the workbook or even open a chart with the
SD values the workbook fails to recalculate sometimes. I use XL97. The
code is provided below. Does anyone have any suggestions?

' This function calculates the positive 2 standard deviation for a
collection of ranges
' Created 6/22/04
' By: Dave Murphy
'
Function SDPDB(ParamArray Levels()) As Variant
Application.Volatile
Dim rng As Variant, S As Double, A As Double, Sig As Double
Dim SigPdB As Double, SigMdB As Double, x As Variant, n As Integer, LA
As Double
A = 0
S = 0
n = 0
For Each rng In Levels()
For Each x In Range(rng.Address)
If Not IsEmpty(x) Then
A = A + 10 ^ (x / 20)
S = S + 10 ^ (x / 10)
n = n + 1
End If
Next x
Next rng
A = A / n
LA = 20 * Application.WorksheetFunction.Log10(A)
Sig = 1 / (n - 1) ^ 0.5 * (S - n * 10 ^ (LA / 10)) ^ 0.5
SDPDB = 20 * Application.WorksheetFunction.Log10(1 + 2 * Sig / A)
End Function

Thanks for any help,
Dave