Discrimination

What is it?

Item discrimination is the degree to which  students with high overall exam scores also got a particular item correct. It is often referred to as Item Effect, since it is an index of an item’s effectiveness at discriminating those who know the content from those who do not.

The item discrimination index is a point biserial correlation coefficient. Its possible range is -1.00 to 1.00. A strong and positive correlation suggests that students who get any one question correct also have a relatively high score on the overall exam. Theoretically, this makes sense. Students who know the content and who perform well on the test overall should be the ones who know the content. There’s a problem if students are getting correct answers on a test and they don’t know the content.

Where do I find it?

In ANGEL, view item discrimination by taking the following steps:

Step 1: Select the test or quiz for which you want to view the item discrimination.
Step 2: Click Reports
Step 3: Click Item Analysis

ANGEL will generate an item analysis based on your class data.

Discrimination appears here:

If you are using the Item Analysis provided by Scanning Operations, discrimination indices are listed under the column head ‘Disc.’

How should I use this information?

As you examine item discrimination, there are a number of things you should consider.

1. Item difficulty! Very easy or very difficult items are not good discriminators. If an item is so easy (e.g., difficulty = 98) that nearly everyone gets it correct or so difficult (e.g., difficulty = 12) that nearly everyone gets it wrong, then it becomes very difficult to discriminate those who actually know the content from those who do not.

(Caution: That does not mean that very easy and very difficult items should be eliminated. In fact, they are fine as long they are used with the instructor’s recognition that they will not discriminate well and if putting them on the test matches the intention of the instructor to either really challenge students or to make certain that everyone knows a certain bit of content.)

2. A poorly written item will have little ability to discriminate.

What should I aim for?

It is typically recommended that item discrimination be at least .20. It’s best to aim even higher. Items with a negative discrimination are theoretically indicating that either the students who performed poorly on the test overall got the question correct or that students with high overall test performance did not get the item correct. Thus, the index could signal a number of problems:

  • There is a mistake on the scoring key.
  • Poorly prepared students are guessing correctly.
  • Well prepared students are somehow justifying the wrong answer.

In all cases, action must be taken! So, items with negative item difficulty must be addressed. Items with discrimination indices less than .20 (or slightly over, but still relatively low) must be revised or eliminated. Be certain that there is only one possible answer, that the question is written clearly, and that your answer key is correct.

How well do I understand it?

Use what you’ve learned about item discrimination to answer the Test Your Understanding questions. For each item, make your choice them click the Continue button.