The following formula is returning the #N/A error:

=SUMPRODUCT(('Retail 2006'!$AE$2:$AE$4000=$D$2)*('Retail
2006'!$C$2:$C$4000<=$N$2),'Retail 2006'!$F$2:$F$4000)

The following formula in other worksheets in the same file is working:

=SUMPRODUCT(('Retail 2006'!$E$2:$E$4000=$D$2)*('Retail
2006'!$C$2:$C$4000<=$N$2),'Retail 2006'!$F$2:$F$4000)

The only difference is the first argument, which is referring to a column
containing store types (Department Stores, Chains etc.) in the worksheet that
isn't working and store codes (C10, C20, C30 etc.) in the one that is. Cell
formats are set to general for both columns.

For your information:

Column C contains months of the year

Column F contains sales

Cells D2 & N2 are linked to combo boxes and display the Store Type and Month
respectively. Again, cell formats are General.

What could be going wrong?

Regards

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R Ormerod