Point-Biserial is a process that examines the students with the highest scores on the test and the students with the lowest scores on the test, comparing them against each other for a particular question.
If the Point-Biserial is a positive number (between .01 and 1.00), it indicates that students who scored in the top 50% of the class on this test answered the questions correctly more often than those who scored in the bottom 50% of the class on the test.
This would be the expected behavior, as students who performed best on the test were more likely to answer that item correctly.
If the Point-Biserial is 0.00, it indicates that all students performed the same on this question (either all answered it correctly or all answered it incorrectly).
If the Point-Biserial is a negative number (between -.01 and -1.00), it indicates that students who scored in the bottom 50% of the class on the test answered the question correctly more often than those who scored in the top 50% of the class on the test.
This would be unexpected behavior. We often see this in questions where students who studied hard were misled by an attractive distractor, while students who didn’t study as hard may have guessed correctly. This generally indicates that the question may warrant review to determine why the lowest-performing students on the test were getting this question correct more often than the highest-performing students.