I have an Excel sheet with three columns: date, person and percentage. I would like to put in a data validation that flags cells if the total for a given data/person combination do not equal 100%. Is that possible?

In other words, in the custom formula of a data validation, I would like to make the following type of formula.

```
=if(sum( cells with a (date = the date on this row, person = person on this row))=1)
```

Is there a function which will return the cells in a range conditioned on certain values, or will sum the cells.

Note that if it is not possible to do two cells, I have no issue adding a cell which combines both values for the purpose of affecting the lookup.