NBME Biostatistics and Epidemiology

Question 1

A dataset has a mean, median, and mode that are all equal. Which of the following BEST describes this distribution?

Accepted Answer

Normal (bell-shaped)

Answer

When mean, median, and mode are equal, the distribution is symmetric and follows a normal (Gaussian) bell-shaped curve.

Question 2

A screening test has 90% sensitivity and 80% specificity. The disease prevalence is 10%. What is the approximate positive predictive value (PPV)?

Accepted Answer

33%

Answer

PPV = (sensitivity × prevalence) / [(sensitivity × prevalence) + (1−specificity)(1−prevalence)] = (0.9×0.1) / (0.9×0.1 + 0.2×0.9) ≈ 33%.

Question 3

Which measure of central tendency is MOST affected by extreme outliers in a dataset?

Accepted Answer

Mean

Answer

The mean uses all values in its calculation, so a single extreme outlier can substantially shift it, unlike the median or mode.

Question 4

A study reports a p-value of 0.03 with alpha set at 0.05. What is the correct interpretation?

Accepted Answer

There is a 3% probability of obtaining results this extreme if the null hypothesis is true

Answer

A p-value represents the probability of observing results at least as extreme as those found, assuming the null hypothesis is true — not the probability that the null is true.

Question 5

The Number Needed to Treat (NNT) is calculated as:

Accepted Answer

1 / Absolute Risk Reduction

Answer

NNT = 1 / ARR; it tells you how many patients must be treated to prevent one adverse outcome compared to control.

NBME Practice Test

NBME Practice Test

NBME Biostatistics and Epidemiology