5. Estimate item difficulty, by determining the percentage of students who got the item
right. The simplest procedure is to base this estimate on only those students included
in the item-analysis groups. Thus, sum the total number in the upper and lower
groups (10 + 10 = 20); sum the number selecting the correct answer. (For item 1.
above, 6 + 2 = 8); divide the first sum into the second and multiply by 100