Because there are different numbers of items on each age interval (and thus varying score ranges), you can compute an average item score for each child and compare those average scores. The average item score is calculated by dividing the total score by the number of scored items on that interval. The chart on page 36 of the ASQ:SE-2 User's Guide includes total scored items per interval, but you will need to adjust if any questionnaires have omitted items.

For example, if a child had a total ASQ:SE-2 score of 225 on the 36 month interval, which has 35 scored items, the average item score is 6.43 (225/35=6.43).