Ignoring N/A Values in a Sum

Chris has a series of worksheets in a workbook, one for each month of the year. On a summary worksheet he wants to sum the values in the same cell on each worksheet. Chris does this by using a formula similar to the following:

=SUM(January:December!B19)

This works fine, except for those instances where one of the B19 cells in the range may contain the value #N/A. In that case, Chris gets #N/A in the result on the summary sheet, as well. What Chris would like is to have the #N/A results ignored for the sum, as if the cells were blank.

There are a couple of ways to approach this problem. Perhaps the best method is to look at the formula used in cell B19 of each month's worksheet. For instance, let's say that the formula on each worksheet looked like this:

=SUM(B1:B18)

You could change the formulas on these individual worksheets so that they took the possibility of #N/A values into account. For instance, the following would work just fine at B19 on each worksheet:

=SUMIF(B1: B18,"<>#N/A")

This causes the sum in cell B19, on each worksheet, to be based on all the non-N/A values in the range. Because of this, you might think you could do this on the summary sheet:

=SUMIF(January:December!B19,"<>#N/A")

This won't work, however, because the SUMIF function is not "three-dimensional" in nature; it cannot be used on a range of worksheets in the manner shown. It is for this reason that the best solution is to go back to the individual values, on each worksheet, that are being tallied on the summary worksheet.

If your formula on the individual month worksheets don't use the SUM function, it is obviously not as easy to change them to use SUMIF. In that case, you may want to "enclose" the existing formula in a check to see if the formula returns an error value. This technique is done this way:

=IF(ISERROR(),0,)

The IF function looks for a True/False value, which is returned by the ISERROR function. Thus, if the formula returns an error value (such as #N/A), then the IF function returns 0, otherwise it returns the result of the original formula. This approach checks for any error result; if you would prefer to have it only check for #N/A results and ignore them, then you can use the following variation:

=IF(ISNA(),0,)

There is a big difference between these IF-based approaches and using the SUMIF approach mentioned earlier in the tip. The SUMIF approach returns a sum for all non-N/A values in the range, but the IF-based approach returns a 0 for the entire sum if there are any #N/A values in the range. This can obviously affect what shows up on your summary sheet, so you will need to determine which approach is best suited to the data you are working with.