Discrimination Index vs Point Biserial

Community Member

Under quiz statistics>Question Breakdown, CANVAS displays a "Discrimination Index" value. But in the Item Analysis output for a given question, that number is actually the "Points Biserial" value and, as far as I can tell, the actual Discrimination Index is not calculated anywhere. I calculated it myself manually and the real Discrimination Index is very different than the "Discrimination Index" reported on CANVAS.

Discrimination index and Point Biserial are different things with very different claculations, Yet, unless I am mistaken CANVAS is using the later but reporting it as the former. What numbers are actually being reported and how are they calculated?

1 Solution
Community Coach

Good afternoon, @Vastib ...

Full disclosure...I am not an instructor, and I do not teach courses in Canvas.  I am a Canvas administrator for our school's Canvas environment.  I'm not sure if this will help or not, but I thought I'd provide you with a couple resources just in case you've not had a chance to review these yet.  You may have seen this already, but there is a small "?" icon next to "Discrimination Index" that you can click on within the Quiz Statistics screen, and it explains "The Discrimination Index Chart":

This metric provides a measure of how well a single question can tell the difference (or discriminate) between students who do well on an exam and those who do not.

It divides students into three groups based on their score on the whole quiz and displays those groups by who answered the question correctly.

There is also a link below these two paragraphs to the following Guide: Once I publish a quiz, what kinds of quiz statisti... - Canvas Community.

Beyond this, unfortunately, I won't be of much help.  There may be others here in the Community that understand this data better than I do.  Another resource for you would be to reach out to the Canvas Help Desk staff.  They might be able to give you more detail on how things are actually calculated.  How do I contact Canvas Support? - Canvas Community

I hope some or all of this will be of help to you.  Sing out if you have any other questions about this...thanks!  Take care, stay safe, and be well.