I have a if sum product function that will work only if all the cells are filled in. How do I add in a formula to exclude any blank cells from the list within this function. 

{=IF(SUMPRODUCT(($O$2:$O$8818=O2)*($I$2:$I$8818>I2))=0,"New","Old")}
I was thinking ERROR, but not sure if that will really work. Again not all rows are filled in and the function will not work until it has all been filled in. Any help would be much appreciated.

Thank you,
Newb