Figure 1.

Errors in adding numbers in Microsoft Excel. Excel's SUM() function, which is used to total all columns in this figure, ignores values that are not numbers. No errors are reported in any of the examples. (a) Two apparently identical sums giving different results. The erroneous sum in the right-hand column is caused by 3.1. having a final decimal point/full stop, and hence being treated as text, and thus processed as zero by SUM. The difference between the column sums may not be noticed by a user, particularly since in normal use they are unlikely to double-check the ‘same’ columns, as used here for illustrative purposes. (b) The ‘show precedents’ feature is one way to help check calculations. It highlights the operands of a cell, but here the precedents for the incorrect total are shown as including the value that has been ignored. Evidently, Excel's notion of ‘precedents’ is the range of possible operands, rather than the actual operands, and therefore the feature is misleading. (c) Through innocent error or intentional mischief, even more unusual column sums can be produced. In the left column, the cell ‘3.1’ is generated by the formula ='3.1', which turns the apparently correct number 3.1 into a string, with value zero as before. In the right column, the cell ‘23’ is actually the number 995, but formatted as ‘23’ using a custom format.