I am looking to sum a field based on two levels of criteria (non numeric) across several wookbooks which are closed. The usual sumifs won't work. Any idea how to get around this?

Use SUMPRODUCT instead. What would your SUMIFS formula look like if it worked?

I would have had this on each of my spreadsheets.

=SUMIFS('Dec15 Pt2'!$S:$S,'Dec15 Pt2'!$N:$N,$A3,'Dec15 Pt2'!$R:$R,$B3)

For each month I have several parts all in different excel files.

You really don't want to use entire columns in a SUMPRODUCT formula so you'll need to limit the rows:

=SUMPRODUCT('C:\path to file\[WORKBOOK.XLSX]Dec15 Pt2'!$S$1:$S$1000,('C:\path to file\[WORKBOOK.XLSX]Dec15 Pt2'!$N$1:$N$1000=$A3)*('C:\path to file\[WORKBOOK.XLSX]Dec15 Pt2'!$R$1:$R$1000=$B3))

Thanks that works a treat. Why is the last part of the formulae a * and not a select? Want to understand how it works.

The last section creates two arrays of TRUE and FALSE values. SUMPRODUCT ignores these values, so to make them count the two arrays are multiplied together. For the multiplication, TRUE is coerced to 1 and FALSE to 0, so you end up with an array of 0 and 1 which is then multiplied by the actual data values before they are then summed up.

Just to add to Debaser's explanation: any arithmetic operation performed on TRUE or FALSE will coerce them to 1 or 0

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