Are your quizzes useful?

This approach is likely to be best for end of course assessment, rather than general self-assessment quizzes.

Difficulty Index

The item difficulty index is one of the most useful, and most frequently reported, item analysis statistics. It is a measure of the proportion of examinees who answered the item correctly; for this reason it is frequently called the p-value. As the proportion of examinees who got the item right, the p-value might more properly be called the item easiness index, rather than the item difficulty. It can range between 0.0 and 1.0, with a higher value indicating that a greater proportion of examinees responded to the item correctly, and it was thus an easier item.

Difficulty index (P score) = (Total true response of the item / Total responses) × 100

Where Total responses= (True response + Wrong response + Not responded)

For a maximum discrimination between high and low achievers, the optimal levels (adjusting for guessing) are:

  • 2 alternatives true and false = .75

  • 3 alternatives multiple-choice = .67

  • 4 alternatives multiple-choice = .63

  • 5 alternatives multiple-choice = .60

Items with difficulties less than 30 percent or more than 90 percent definitely need attention. Such items should either be revised or replaced.

Discriminating index

The item discrimination index is a measure of how well an item is able to distinguish between examinees who are knowledgeable and those who are not, or between masters and non-masters.

Total score for each individual student is calculated and arranged in descending order from highest score to lowest score. Upper 1/3 students are selected to include in higher group (H) and lower 1/3 students are selected to include in lower group (L).

Item Discriminating index is calculated by the following formula:

Discriminating index (D-score) = (HT-LT / T) × 2


  • HT = Number of correct responses in Upper Group,

  • LT = Number of correct responses in Lower Group

  • T = Total number of responses in both group.

Accessing the D-score:

  • Negative D-score - Unacceptable - check item for error

  • 0% - 24% - Usually unacceptable - might be approved

  • 25% - 39% - Good item

  • 40% - 100% - Excellent item

The maximum item discrimination difference is 100 percent. This would occur if all those in the upper group answered correctly and all those in the lower group answered incorrectly. Zero discrimination occurs when equal numbers in both groups answer correctly. Negative discrimination occurs when more students in the lower group than the upper group answer correctly (indicating an error in the analysis).