Hi Jim,

Try this sample out:

Referring to a Field in the Previous Record or Next Record
    http://support.microsoft.com/?id=210504

Tom
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I want to set up an expression that calculates the difference between the
last entered value in a table and the value entered on a form (which is based
on the same record set).  
i.e.  This week's meter reading minus last week's meter reading.  
I know I'll use a macro, but cannot seem to figure out how to select the
values for this expression.  Any ideas?
Thanks,
Jim