User guide

Triton Data Collection System

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Discrimination Index

The discrimination index for a particular question is based on the point biserial correlation coefficient, which measures

the correlation between the correct answer on a question and the total test scores of the active participants.

To calculate this statistic, you must first determine the following:

• Total number of Participants that answered the question correctly

• Total number of Participants that answered the question incorrectly

A Participant that has not responded to a question is considered to have answered incorrectly.

• Mean total score of the group of Participants that have answered the question correctly*

• Mean total score of the group of Participants that answered the question incorrectly*

• Mean total score of all active Participants*

• Standard Deviation

standard deviation(Ȫ) = Ҁ ( ѷ ((x - ȣ)²) / n)

• x = a Participant’s score

• ȣ = mean total score

• n = total number of Participants

The discrimination index can then be computed with the following formula:

discrimination index = ((Mp-Mq) / Ȫ) * Ҁ ((Np*Nq)/((Np+Nq)* (Np+Nq-1))

• Mp = mean total score of the group of Participants that answered question the correctly*

• Mq = mean total score of the group of Participants that answered the question incorrectly*

• Np = total number of Participants that answered the question correctly

• Nq = total number of Participants that answered the question incorrectly

• Ȫ = standard deviation of scores for the entire set

1. Gene V. Glass and Kenneth D. Hopkins, Statistical Methods in Education and Psychology, 3rd ed. (Needham

Heights, MA: Allyn and Bacon,1996), 134-135.