Biostatistics is one of the most predictable yet high-scoring sections of the USMLE Step 1. Questions frequently test concepts such as sensitivity, specificity, predictive values, risk reduction, bias, and study design.
Below are 25 high-yield USMLE Step 1 biostatistics practice questions with clear explanations, designed to mirror real exam logic and help you answer questions quickly under time pressure.
Question 1
A screening test for disease X has 90% sensitivity and 80% specificity. In a population of 1,000 people, 100 have the disease. How many false negatives are expected?
A) 10
B) 20
C) 80
D) 90
Answer: A) 10
Explanation:
Sensitivity = TP / (TP + FN)
90% of 100 diseased patients are detected → 90 true positives.
False negatives = 100 − 90 = 10.
Question 2
In a population where disease prevalence is low, what happens to the positive predictive value (PPV)?
A) Increases
B) Decreases
C) Stays same
D) Equals sensitivity
Answer: B) Decreases
Explanation:
Low prevalence → more false positives relative to true positives → PPV decreases.
Question 3
A drug reduces mortality from 10% to 5%. What is the absolute risk reduction (ARR)?
A) 5%
B) 50%
C) 2%
D) 0.5%
Answer: A) 5%
Explanation:
ARR = 10% − 5% = 5%.
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Question 4
Using Question 3 data, what is the number needed to treat (NNT)?
A) 5
B) 10
C) 20
D) 50
Answer: C) 20
Explanation:
NNT = 1 / ARR
ARR = 0.05 → 1 / 0.05 = 20
Question 5
Which study design is best to determine incidence?
A) Case-control
B) Cross-sectional
C) Cohort
D) Case series
Answer: C) Cohort
Explanation:
Cohort studies follow exposed groups over time → measure incidence.
Question 6
A study selects patients with lung cancer and looks backward to assess smoking history. Study type?
A) Cohort
B) Case-control
C) Cross-sectional
D) RCT
Answer: B) Case-control
Question 7
Which bias occurs when participants are not representative of the general population?
A) Recall bias
B) Selection bias
C) Confounding
D) Observer bias
Answer: B) Selection bias
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Question 8
A p-value of 0.03 means:
A) 3% chance null is true
B) 3% probability results due to chance if null is true
C) 97% confidence null is false
D) Study is invalid
Answer: B)
Explanation:
P-value = probability of observing results assuming null hypothesis is true.
Question 9
A 95% confidence interval includes 1 for relative risk. Interpretation?
A) Statistically significant
B) Not statistically significant
C) High power
D) High bias
Answer: B)
Explanation:
If CI for RR includes 1 → no significant association.
Question 10
Type I error represents:
A) False negative
B) False positive
C) Low power
D) Sampling bias
Answer: B)
Explanation:
Type I error = rejecting true null (false positive).
Question 11
Power of a study increases when:
A) Sample size decreases
B) Variability increases
C) Sample size increases
D) Alpha decreases
Answer: C)
Question 12
Relative risk is best calculated in:
A) Case-control
B) Cohort
C) Cross-sectional
D) Ecologic
Answer: B)
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Question 13
Odds ratio approximates relative risk when:
A) Disease is common
B) Disease is rare
C) Sample small
D) Study biased
Answer: B)
Question 14
A confounder must:
A) Be caused by exposure
B) Be caused by outcome
C) Be associated with both exposure and outcome
D) Be random
Answer: C)
Question 15
Which increases specificity?
A) Lower cutoff value
B) Raise cutoff value
C) Increase prevalence
D) Increase sensitivity
Answer: B)
Explanation:
Higher threshold → fewer false positives → higher specificity.
Question 16
A test with high sensitivity is best for:
A) Confirming disease
B) Screening
C) Prognosis
D) Measuring incidence
Answer: B)
Explanation:
High sensitivity → few false negatives → good screening tool.
Question 17
If prevalence increases, NPV:
A) Increases
B) Decreases
C) Same
D) Equals PPV
Answer: B)
Question 18
Standard deviation measures:
A) Accuracy
B) Spread of data
C) Bias
D) Central tendency
Answer: B)
Question 19
Normal distribution mean = median = mode when:
A) Skewed
B) Symmetric
C) Biased
D) Multimodal
Answer: B)
Question 20
Which is most resistant to outliers?
A) Mean
B) Median
C) Mode
D) SD
Answer: B)
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Question 21
A forest plot shows CI crossing vertical line. Meaning?
A) Significant
B) Not significant
C) High power
D) High effect
Answer: B)
Question 22
Double-blinding reduces:
A) Recall bias
B) Observer bias
C) Selection bias
D) Confounding
Answer: B)
Question 23
Which is the gold standard study design?
A) Cohort
B) Case-control
C) RCT
D) Cross-sectional
Answer: C)
Question 24
Incidence rate equals:
A) Existing cases / population
B) New cases / population at risk
C) Deaths / population
D) Total cases / time
Answer: B)
Question 25
If beta = 0.2, power equals:
A) 20%
B) 50%
C) 80%
D) 95%
Answer: C)
Explanation:
Power = 1 − β
1 − 0.2 = 0.8 (80%)
❓ Frequently Asked Questions
Is biostatistics high yield for USMLE Step 1?
Yes. Biostatistics questions are predictable and frequently tested, making them a scoring opportunity.
How many biostatistics questions appear on Step 1?
Biostatistics concepts appear across multiple blocks, often integrated with ethics and clinical scenarios.
Are these questions suitable for international medical graduates (IMGs)?
Yes. These MCQs follow standard USMLE logic used for both US and international candidates.