The r-Biserial is a correlation coefficient relating performance on an item and performance on the total assignment. The r-Biserial correlation coefficient is an index of discrimination that measures the extent to which students who score high on the assignment tend to get the item correct and those who score low tend to get the item incorrect.
where = mean grade of assignments on which this item was answered incorrectly. = mean grade of assignments on which this item was answered fully correctly. p = p-Value q = 1-p = standard deviation for assignment grades Y = height of the standardized normal distribution at the point z, where P(z'<z)=q and P(z'>z)=p Note: Since the r-Biserial is scaled by a parameter that is based on a normal distribution, it can result in values outside of -1 to 1 if the scores are not normally distributed. |