Error that occurs when each number in a set of numbers is rounded according to some rule with the result that the total of the rounded numbers is not equal to the rounded version of the original total. For example, rounded to the nearest 10, the counts 21, 22, 23, and 24 each become 20. However, 4×20=80 whereas the total of the unrounded numbers is 90.

Rounding errors are most often noticed when a set of percentages that should sum to 100% actually sums to something different (e.g. 99% or 101%).