A simple way for remembering a percentage is that it shows a part of the whole. Cumulative percentages add a percentage from one period to the percentage of another period. This calculation is important in statistics because it shows how the percentages add together over a time period. For example, if a person has data on how many times it snowed in January, he would then add the data from February to tell the cumulative percentage of days of snow for January and February.

Add together the number of times the event occurred. For example, in January it snowed 10 days and February it snowed 15 days. The total times the event occurred is 25 days.

Add together the different sample sizes. January has 31 days and February has 28 days for the time period examined. Therefore, the total sample size is 59 days.

Divide the number of times the event occurred by the total sample size to find the cumulative percentage. In the example, 25 days divided by 59 days equals 0.423729 or 42.3729 per cent.