The following example checks to determine if new data entered in a field is within 10 percent of the value of the original data. If the change is greater than 10 percent, the OldValue property is used to restore the original value. This procedure could be called from the BeforeUpdate event of the control that contains data you want to validate.
Sub Validate_Field()
Dim curNewValue As Currency
Dim curOriginalValue As Currency
Dim curChange As Currency, strMsg As String
curNewValue = Forms!Products!UnitPrice
curOriginalValue = Forms!Products!UnitPrice.OldValue
curChange = Abs(curNewValue - curOriginalValue)
If curChange > (curOriginalValue * .1) Then
strMsg = "Change is more than 10% of original unit price. " _
& "Restoring original unit price."
MsgBox strMsg, vbExclamation, "Invalid change"
Forms!Products!UnitPrice = curOriginalValue
End If
End Sub