HE Instructor Knowledge Base URL: http://cmg.screenstepslive.com/s/MacmillanMedia/m/LaunchPadIM/l/126703-explaining-difficulty-and-discrimination
Explaining Difficulty and Discrimination
From the same screen as in the section "Curving: Changing Scores and Explaining Grade Redistribution," scroll further down to the point where you see "difficulty" and "discrimination". Start with difficulty - click on the word "difficulty" to sort questions by difficulty percentage.
As you hover with your mouse over one of the bars for the question, you'll see the number of students who've answered the question, and the percentage of students who got it right and wrong. Green represents correct, and red represents incorrect.
You can also click on the + symbol to see the percentage of students who selected each distactor/answer option.
Now click on the word "discrimination." Discrimination is calculated as the correlation between how well the students answered the question and how well the students did on the assessment. Hovering over one of the bars displays a correlation score.
The discrimination score that instructors desire is .2 or above. The closer it gets to 1 the better the question correlates to the whole assignment. This analysis is called "discrimination analysis" and it is a scientific method for indicating student performance on a particular item as compared to student performance on the assessment as a whole. A low positive or zero correlation may mean that this is a basic question, one that you expect all students to correctly answer. A negative correlation or low correlation is undesirable because it means performance on the question was not consistent with performance on the assessment as a whole.