Compute Discrimination Index
In the context of norm referenced tests, the discrimination index (DI) of a question indicates how well the question helps distinguish those who do well on the total test and those who perform poorly.
DI = (Number of correct answers in HS - Number of correct answers in LS) / size of each group
DI = (8 - 2) / 10 = .60 (based on previous example)
The discrimination index ranges from -1.0 to + 1.0. A positive index means that getting that particular question right positively correlates with higher total test scores (Worthen et al., 1999). A negative index means students who get the question right usually score lower on the test than those who get the question wrong. Therefore, a question that has a negative index needs further attention.
Hopkins and Stanley (1981) provide the following guidelines for the discrimination index (as cited in Worthen et al., 1999).
| Index | Evaluation |
| Below .10 | Poor item- delete or revise |
| .10 - .19 | Subject to improvement |
| .20 - .29 | Reasonable good question |
| .30 - .39 | Good question |
| .40 - above | Very good question |
The above discrimination index is .60, which indicates a very good question.
The discrimination index can also be calculated for each question option in the previous example. For practice, verify your answers with the following discrimination index for each option:
- DI for option a = -.10
- DI for option b = -.30
- DI for option c = .60
- DI for option d = .00
- DI for option e = -.20
Remember: the option that has the largest negative index is the one that is most frequently selected by low scorers compared to higher scorers. This provides useful information about misconception and for remediation.
Reference
- Worthen, B. R., White, K. R., Fan, X., & Sudweeks, R. R. (1999). Measurement and assessment in schools (2nd ed.). New York, NY: Addison Wesley Longman.