What is PBCC (Point-Biserial Correlation)?
Understanding how to interpret and use information based on student test scores is as important as knowing how to construct a well-designed test. Using statistical information to review your multiple-choice test can provide useful information.
On the Item Analysis table of the IRAT Dashboard, notice the last column, "PBCC".
PBCC , also referred to as the Point-Biserial Correlation shows the relationship between how well students did on the item and their total test score.
The range is from –1.00 to 1.00.
The higher the value, the more discriminating the item. A highly discriminating item indicates that the students who had high tests scores got the item correct whereas students who had low test scores got the item incorrect.Items with discrimination values near or less than zero should be removed from the test. This indicates that students who overall did poorly on the test did better on that item than students who overall did well. The item may be confusing for your better scoring students in some way.
The acceptable range: 0.20 or higher
The ideal value: The closer to 1.00 the better
Calculation:
where,
= The mean total score for persons who have responded correctly to the item
= The mean total score for all persons
= The difficulty value for the item
= p-1
= The standard deviation of total test scores
How does InteDashboard calculate the PBCC of Point-spreading type of IRAT?
InteDashboard treats partials scores as zero. This means that if a student in a confidence-based test gets 50% points in the question, we treat it as wrong, because they are not 100% correct.