Epidemiology: secondary attack rate: practice question

A PSM question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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PSM · Epidemiology: secondary attack rate

Sample item

A household has a primary case of measles and 9 other members, of whom 1 is immune. Within the incubation period 4 of the susceptible contacts develop measles. What is the secondary attack rate?

A40 percentB44 percentC50 percentD10 percent

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

C. 50 percent

Secondary attack rate is the number of secondary cases divided by the number of susceptible contacts, excluding the primary case. Susceptible contacts are 9 minus 1 immune, which is 8, and 4 divided by 8 gives 50 percent.

Option by option

A. 40 percent
40 percent comes from dividing 4 by 10, wrongly counting the primary case.
B. 44 percent
44 percent comes from dividing by 9 and not excluding the immune member.
C. 50 percent (correct)
Correct. 4 secondary cases among 8 susceptible contacts is 50 percent.
D. 10 percent
10 percent is not derived from the data.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this PSM question

What is the answer to this PSM question?

The best answer is C, 50 percent. Secondary attack rate is the number of secondary cases divided by the number of susceptible contacts, excluding the primary case.

Why is "40 percent" not the answer?

40 percent comes from dividing 4 by 10, wrongly counting the primary case.

Why is "44 percent" not the answer?

44 percent comes from dividing by 9 and not excluding the immune member.

Why is "10 percent" not the answer?

10 percent is not derived from the data.

What concept does this question test?

It tests Epidemiology: secondary attack rate in PSM. Susceptible contacts are 9 minus 1 immune, which is 8, and 4 divided by 8 gives 50 percent.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Epidemiology: secondary attack rate?

Practise biostatistics with small calculations until sensitivity, predictive values and risk measures are automatic. Then use the flashcards and the other PSM questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.