When I use SUMPRODUCT formula, especially to look up data from
different workbooks and return to a different workbook, I tend to
have
problem later on when I work on it or save it to a different file.
All
my other formulas e.g IF, VLOOK never been messed up when I work on
the file or save it to a different file.

Is there anyway I could protect this SUMPRODUCT formula or any reason
why it get messed up so easily?
Thanks


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