Biostatistics: snowball sampling: 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 · Biostatistics: snowball sampling

Sample item

Researchers wish to study HIV risk behaviour among people who inject drugs in a city, where no list of such persons exists and members are reluctant to come forward. Which sampling approach is most practical?

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

The answer and the reasoning

C. Snowball sampling through referrals from initial participants

Snowball sampling is a non-probability method in which initial participants refer other members of a hidden population. It is used when no sampling frame exists. Its drawback is selection bias, since the sample depends on social networks.

Option by option

  • A. Systematic sampling of every 10th household

    Household sampling would reach very few members of this hidden group.

  • B. Stratified random sampling by age group

    Stratified sampling also needs a frame from which to draw within strata.

  • C. Snowball sampling through referrals from initial participants (correct)

    Correct. Referral chains reach hidden populations without a list.

  • D. Simple random sampling from a sampling frame

    Simple random sampling needs a complete sampling frame, which is absent.

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.

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Questions about this PSM question

What is the answer to this PSM question?

The best answer is C, Snowball sampling through referrals from initial participants. Snowball sampling is a non-probability method in which initial participants refer other members of a hidden population.

Why is "Systematic sampling of every 10th household" not the answer?

Household sampling would reach very few members of this hidden group.

Why is "Stratified random sampling by age group" not the answer?

Stratified sampling also needs a frame from which to draw within strata.

Why is "Simple random sampling from a sampling frame" not the answer?

Simple random sampling needs a complete sampling frame, which is absent.

What concept does this question test?

It tests Biostatistics: snowball sampling in PSM. It is used when no sampling frame exists. Its drawback is selection bias, since the sample depends on social networks.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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 Biostatistics: snowball sampling?

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.