Hello,

In VBA the operator "-" is not defined on ranges.

A workaround:
Sub calcStdev()

Dim myRangeAct As Range
Dim myRangeMod As Range
Dim a(1 To 100) As Double
Dim i As Long

Set myRangeAct = Range(Cells(1, 1), Cells(100, 1))
Set myRangeMod = Range(Cells(1, 2), Cells(100, 2))

Debug.Print Application.WorksheetFunction.StDevP(myRangeAct)
Debug.Print Application.WorksheetFunction.StDevP(myRangeMod)

For i = 1 To 100
a(i) = myRangeAct(i) - myRangeMod(i)
Next i

Debug.Print Application.WorksheetFunction.StDevP(a)

End Sub

Regards,
Bernd