Statistics for People Who (Think They) Hate Statistics

by Neil J. Salkind

Select a class interval so that 10 to 20 such intervals cover the entire range of data. A convenient way to do this is to compute the range and then divide by a number that represents the number of intervals you want to use (between 10 and 20). In our example, there are 50 scores, and we wanted 10 intervals: 50/10 = 5, which is the size of each class interval. If you had a set of scores ranging from 100 to 400, you could start with an estimate of 20 intervals and see if the interval range makes sense for your data: 300/20 = 15, so 15 would be the class interval. Begin listing the class interval with a multiple of that interval. In our frequency distribution of reading comprehension test scores, the class interval is 5, and we started the lowest class interval at 0. Finally, the interval made up of the largest scores goes at the top of the frequency distribution.