What is the purpose of an item discrimination index and how is it calculated?

Question

What will be an ideal response?

Answer 1

• Because inferences regarding the meaning of high test scores versus low test scores will be made by the users of a test, it is important to obtain a measure of how well each item separates those test takers who demonstrate a high degree of skill, knowledge, attitude, or personality characteristic from those who demonstrate little of the same skill, knowledge, attitude, or personality characteristic.

• If a test is well constructed, each item on it (or on the subtest if the test is designed to measure multiple concepts) should be a measure of the concept of interest. If this is the case, there should be a statistical relationship between how an individual answers a particular item and his or her overall score on the test.

• One of the ways that test developers evaluate this is by calculating a discrimination index, which compares the performance of those who obtained very high test scores (the upper group [U]) with the performance of those who obtained very low test scores (the lower group [L]) on each item. Calculating the percentage of test takers in each group who responded correctly and then obtaining the difference between the two percentages creates the discrimination index.
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D = U – L.
• The upper group and lower group are formed by ranking the final test scores from lowest to highest and then taking the upper third and the lower third to use in the analysis.

• After calculating a D value for each item, test developers look for items that have high positive numbers. Negative numbers indicate that those who scored low on the test overall responded to the item correctly and that those who scored high on the test responded incorrectly. Low positive numbers suggest that nearly as many people who had low scores responded correctly, as did those who had high scores.