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=  M p – M q         St

To calculate the rpbi for each item, use the following formula:

rpbi

pq

rpbi =

point-biserial correlation coefficient

Mp =

whole-test mean for students answering item correctly (i.e., those coded as 1s)

Mq =

whole-test mean for students answering item incorrectly (i.e., those coded as 0s)

St =

standard deviation for whole test

p =

proportion of students answering correctly (i.e., those coded as 1s)

q =

proportion of students answering incorrectly (i.e., those coded as 0s)

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