Step 4 Determine the lowest class interval. Our lowest score is 51, so we select 50 for the beginning point of the lowest interval (it is a multiple of our interval width). Because i ¼ 5, we add 4 (i.e., 5  1) to this point to obtain our lowest class interval: 50–54. (If we had added 5, we would have an interval width of 6. Remember: i reflects the number of score values in a class interval.)