Item Discrimination I
The single best measure of the effectiveness of an item is its ability to separate students who vary in their degree of knowledge of the material tested and their ability to use it. If one group of students has mastered the material and the other group had not, a larger portion of the former group should be expected to correctly answer a test item. Item discrimination is the difference between the percentage correct for these two groups.
Item discrimination can be calculated by ranking the students according to total score and then selecting the top 27 percent and the lowest 27 percent in terms of total score. For each item, the percentage of students in the upper and lower groups answering correctly is calculated. The difference is one measure of item discrimination (IDis). The formula is:
IDis = (Upper Group Percent Correct) – (Lower Group Percent Correct)
Item #1 in the attached report would have an IDis of
100% - 62.5% = 37.5% (or .375 as a decimal).
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, a highly undesirable condition, occurs when more students in the lower group then the upper group answer correctly. Item #6 on the attached report has a negative IDis.
The following levels may be used as a guideline for acceptable items.
|Negative IDis||Unacceptable – check item for error|
|0% - 24%||Usually unacceptable – might be approved|
|25% - 39%||Good item|
|40% - 100%||Excellent item|