Biostatistics: power of a study: 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: power of a study

Sample item

A trial protocol states that the study has 80% power to detect a 10 mmHg difference in blood pressure between the arms. What does this mean?

AThe probability of a type I error is 80%BIf a true difference of 10 mmHg exists, there is an 80% chance the study will detect it as significantCThe result will be clinically important in 80% of participantsDThere is an 80% probability that the null hypothesis is true

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

The answer and the reasoning

B. If a true difference of 10 mmHg exists, there is an 80% chance the study will detect it as significant

Power is 1 minus beta, the probability of correctly rejecting a false null hypothesis. With 80% power the beta error, the risk of missing a real difference of the stated size, is 20%. Power rises with larger sample size, larger effect size and a higher alpha.

Option by option

A. The probability of a type I error is 80%
The type I error is alpha, conventionally 5%, not the power.
B. If a true difference of 10 mmHg exists, there is an 80% chance the study will detect it as significant (correct)
Correct. Power is the chance of detecting a real effect of the stated size.
C. The result will be clinically important in 80% of participants
Power concerns the study as a whole, not the proportion of participants who benefit.
D. There is an 80% probability that the null hypothesis is true
Power does not describe the probability that the null hypothesis is true.

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 B, If a true difference of 10 mmHg exists, there is an 80% chance the study will detect it as significant. Power is 1 minus beta, the probability of correctly rejecting a false null hypothesis.

Why is "The probability of a type I error is 80%" not the answer?

The type I error is alpha, conventionally 5%, not the power.

Why is "The result will be clinically important in 80% of participants" not the answer?

Power concerns the study as a whole, not the proportion of participants who benefit.

Why is "There is an 80% probability that the null hypothesis is true" not the answer?

Power does not describe the probability that the null hypothesis is true.

What concept does this question test?

It tests Biostatistics: power of a study in PSM. With 80% power the beta error, the risk of missing a real difference of the stated size, is 20%. Power rises with larger sample size, larger effect size and a higher alpha.

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: power of a study?

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.