1. Negative Marking
Guidance for Test Takers:
Select the option that seems to you the
most likely to be correct for each question.
Only one option is correct per question.
Three marks are awarded for every correct
selection, minus one for every incorrect
selection.
Random guesses are equally likely to result
in a lower or higher test score, so it is safer
to skip questions unless you feel that you
can at least make an “educated” guess.
2. Rationale:
Any randomly selected option is three
times more likely to be incorrect than
it is to be correct.
Therefore, by awarding +3 for every
correct selection and -1 for every
incorrect selection we ensure that
Test Takers have nothing to gain, on
average, by making purely random
selections.
3. Negative marking within multiple-
choice tests has two benefits:
1) Since pure guesswork is no
longer advantageous, the
test scores are more realistic,
2) less guesswork means the
test scores are more reliable.
5. A good test should yield a score that is
proportional to each test taker's knowledge,
comprehension, or aptitude; whatever it is
the test has been designed to assess.
6. However, if test takers guess some of the
answers this will lead to unrealistically high
test scores if there is no negative marking.
7. If each question has 4 answer options, the expected score
for a test taker who has zero knowledge, comprehension,
or aptitude is 25%. This is an average score, since each
test taker's actual score will be variable.
8. There are two distinct problems here.
The first problem is that pure guesswork leads
to unrealistically high test scores. This is not
a major problem, because the scores can be
scaled down to compensate for the inflation.
The second - and much more serious - problem
is that pure guesswork leads to variable test
scores. This means that traditional multiple-
choice tests without negative marking are
inherently unreliable!
9. To illustrate the problem,
consider a test consisting
of 25 multiple-choice
questions, each with 4 25 Qs
answer options.
✔
Suppose that a particular
5 Qs
test taker knows the
right answer to the first
5 questions, and that
they make purely random ?
guesses for the remaining 20 Qs
20 questions.
10. If each question is worth
3 marks, the test taker
will get 15 marks for the
first 5 questions. 25 Qs (3 marks per Q)
(You'll soon see why we ✔
need to have 3 marks per
5 Qs 5 x 3 = 15 marks
question rather than just
one mark.)
?
20 Qs
11. If they then make
random guesses for the
remaining 20 questions,
on average they will get 25 Qs (3 marks per Q)
5 of them right, which
✔
will give them another 15 5 Qs 5 x 3 = 15 marks
marks.
?
20 Qs 5 x 3 = 15 marks
12. This will give them 30
marks altogether, on
average, out of the 75
marks that are available. 25 Qs (3 marks per Q)
So their expected score ✔
is 30 out of 75, which is
5 Qs 5 x 3 = 15 marks
2/5, or 40%.
(Their actual score may Score = 30 out of 75
(expected) = 40%
of course be greater
than or less than 40%,
?
20 Qs 5 x 3 = 15 marks
with equal probability.)
13. In other words, a test taker who knows the answers to
20% of the questions has an expected score of 40%,
since on average they will guess the right answers to a
quarter of the remaining 80% of the questions.
14. Now we get to the negative marking!
If we deduct one mark for every question
answered incorrectly these negative marks
will (on average) cancel out the positive
marks gained through lucky random guesses.
The idea is that for each lucky guess that
adds 3 marks to the score, there will be (on
average) 3 unlucky guesses that each
deducts one mark from the score.
Consequently it is not advantageous (on
average) for test takers to engage in pure
guesswork.
15. So in our example the
expected score is now
20%, which is in line with
the profile of a "good" 25 Qs (3 marks or -1 mark per Q)
test (which we saw in the
✔
5th slide). 5 Qs 5 x 3 = 15 marks
Research has shown that
negative marking does in Score = 15 out of 75
practice discourage test (expected) = 20%
takers from engaging in ?
pure guesswork. 20 Qs 5 x 3 = 15 marks
This means that the
actual scores will be less 15 x -1 = -15 marks
variable, and therefore
the use of negative
marking increases test
reliability!
16. To conclude, here’s the third slide again...
Negative marking within multiple-
choice tests has two benefits:
1) pure guesswork is no longer
advantageous, therefore the
test scores are more realistic,
2) less guesswork means the
test scores are more reliable.
17. Thank you for viewing this presentation!
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