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VOTED 2007 BEST TEACHING TIP FOR GEEKS!
Have you ever wondered what those biserial correlation scores mean on
your graded scantron reports? Can you use them to help decide which
questions should be kept and which ones should be thrown out? Well
you can, and this week’s tip will explain how. Thanks to fellow MTI
Coordinator Erica Suchman from the College of Veterinary Medicine &
Biomedical Sciences for passing this tip along!
Are you confused about what the biserial correlation number on a
Scantron report is, and what you're supposed to do with it? This should
help. A biserial correlation is a statistical measure indicating the
strength of the relationship between the right answer for each question
on a multiple-choice, fill-in-the-bubble exam, relative to the total
number of correct answers for all other questions on the same exam. It
is arrived at by comparing how well students did answering one question
relative to how well they did answering all the questions.
Implicit is that that those who performed well on all questions should
have performed well on the one being measured; those who did poorly on
all should have done poorly on the one being measured. For each
question, the closer to +1 the coefficient is, the better the question
fits with the other questions; the closer to 0, the lower—or more
suspect—the value of the question. Negative correlations are often
an indication that the question may need to be reverse coded. Properly
interpreted, a biserial correlation coefficient will help whomever
designed an exam determine whether a question should be retained,
revised or removed from current and future exams.
It's a matter of percentages. Here's what a Scantron report will look
like.
| Exam Summary |
| Form A: 5 students, mean score 52.11 (range 33.33 to 78.78) |
Here's how to interpret the Scantron report. A question returning with:
A high percentage of students scoring a correct answer, paired with a
biserial correlation greater than +.10 (see question 1 above), indicates
a fundamentally good question. Those who did well on the exam overall,
did well on this question. Such questions have a high value and should
always be retained.
A low percentage of students scoring a correct answer, paired with a
biserial correlation greater than+.10 (see question 3 above), also
indicates a fundamentally good question—an excellent one, as a matter of
fact—the distinction being, the question is more difficult. Only the
best students should be expected to answer it correctly. Such questions
have a high value and should always be retained.
A high percentage of students scoring a correct answer paired with a
biserial correlation greater than -.10 but less than +.10 suggests a
relatively easy question or, one that may be simply too easy for the
test. A mediocre percentage—say between 50% and 70%—of students scoring
an answer correctly, but paired with a similar biserial correlation, may
be removed or reworded. [Note: No hard and fast rules here—this is an
instructor call.]
A low percentage of students scoring a correct answer paired with a
biserial correlation less than -.10 (see question 2 above) indicates a
question that may be keyed wrong or need to be reverse coded. There is
something wrong, either in wording, scoring, syntax, and/or content.
Those who did poorly on the exam are more likely to score a correct
answer than those who did well. Without revision, the question has no
value and the instructor should consider its removal. [Note: the
instructor should check to see that the answer was properly keyed in to
the computer—unintentional input errors can skew results as well.]
Contributors: TILT thanks Dr. Erica Suchman, Department of Microbiology,
Immunology and Pathology and Prof. Jim zumBrunnen, Department of
Statistics at Colorado State University for assisting with this tip.
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