Turning Technologies Triton Web - Manual (Page 44)

Triton Data Collection System 42
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Discrimination Index
1
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.