Question Discrimination
Tests provide important feedback to both students and instructors. The question discrimination index sometimes referred to by the name question effect, can be invaluable in aiding this feedback. Briefly, the discrimination index is a kind of correlation that describes the relationship between responses to a question and the total score on a test. For most classroom tests, instructors want to provide an opportunity to students to demonstrate what they know and can do. Experience tells us that most groups of students will be variable in their performance. One purpose of tests is to differentiate among students in terms of their ability and preparation. We can check how well each question helps us to do that by examining question discrimination.
- As a correlation, question discrimination can theoretically take values between -1.00 and +1.00. In practical terms values for most classroom tests range between near 0.00 to values near .90.
- If a question is very easy so that nearly all students answered correctly, the questions discrimination will be near zero. Extremely easy questions cannot distinguish among students in terms of their performance.
- If a question is extremely difficult so that nearly all students answered incorrectly, the discrimination will be near zero.
- The most effective questions will have moderate difficulty and high discrimination values. The higher the value of discrimination is, the more effective it is in contribution to the discrimination among students in terms of their performance.
- Questions having low or negative values of discrimination need to be reviewed very carefully for confusing language or an incorrect key. If no confusing language is found then the course design for the topic of the question needs to be critically reviewed.
- A high level of student guessing on questions will result in a question discrimination value near zero.