Biostatistics Practice Questions
20 free Biostatistics practice questions for the USMLE Step 1. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
A dataset has the following values: 2, 3, 3, 4, 10. Which measure of central tendency is most affected by the extreme value?
- A Mean
- B Median
- C Mode
- D Interquartile range
Correct answer: Mean
The mean is strongly influenced by extreme values, while the median and mode are less sensitive.
In a positively skewed distribution, which relationship between mean, median, and mode is expected?
- A Mean < Median < Mode
- B Mode < Median < Mean
- C Median < Mode < Mean
- D Mean = Median = Mode
Correct answer: Mode < Median < Mean
In positive skew, the long right tail pulls the mean upward, with mode lowest and mean highest.
A study reports that smokers have a lung cancer incidence of 30 per 1,000, while nonsmokers have 5 per 1,000. What is the relative risk?
- A 5
- B 6
- C 0.17
- D 25
Correct answer: 6
Relative risk = 30/5 = 6, indicating smokers are six times more likely to develop lung cancer.
A screening test incorrectly labels many healthy people as having a disease. Which statistic is reduced?
- A Sensitivity
- B Specificity
- C Incidence
- D Prevalence
Correct answer: Specificity
Specificity decreases when the test yields more false positives.
Which measure is most appropriate for describing the central value of ordinal data such as pain scales?
- A Mean
- B Median
- C Standard deviation
- D Variance
Correct answer: Median
The median is appropriate for ordinal data because values represent ranked categories, not equal intervals.
A researcher reports that the median age of patients is 37. What does this imply?
- A The average age of patients is 37
- B Half the patients are older than 37
- C The most commonly occurring age is 37
- D The distribution of ages is normal
Correct answer: Half the patients are older than 37
The median is the 50th percentile, meaning half the sample is above and half below that value.
A high standard deviation in a dataset indicates:
- A The mean is inaccurate
- B Greater variability around the mean
- C Data are clustered tightly
- D The distribution must be skewed
Correct answer: Greater variability around the mean
Standard deviation measures how spread out the values are; higher values imply more spread.
A cohort study finds a relative risk of 1.0. What does this indicate?
- A Exposure increases the risk of disease
- B Exposure decreases the risk of disease
- C No link between exposure and disease
- D The study must be invalid
Correct answer: No link between exposure and disease
A relative risk of 1 means the exposed and unexposed groups have equal risk.
A case-control study reports an odds ratio of 4. This means:
- A The odds of disease are four times higher
- B The exposure reduces disease risk by 75%
- C The disease is relatively rare
- D The odds are equal between the two groups
Correct answer: The odds of disease are four times higher
An odds ratio of 4 means the exposed group has four times the odds of disease.
The interquartile range (IQR) represents the:
- A Middle 50% of data
- B Entire data range
- C Lowest 50% of data
- D Difference between mean and median
Correct answer: Middle 50% of data
IQR is Q3 – Q1 and captures the central 50% of the data, reducing influence of outliers.
A distribution has mean = 10 and median = 5. Which statement is most likely?
- A The distribution is symmetric
- B The distribution is positively skewed
- C The distribution is negatively skewed
- D The sample size is too small
Correct answer: The distribution is positively skewed
A mean higher than the median suggests a right-skewed distribution.
A test with high sensitivity is most useful for:
- A Confirming disease
- B Screening for disease
- C Measuring treatment effect
- D Monitoring disease severity
Correct answer: Screening for disease
High sensitivity means few false negatives, making it ideal for screening.
A disease has a prevalence of 5%. Which value is most directly affected by prevalence?
- A Sensitivity
- B Specificity
- C Positive predictive value
- D Likelihood ratio
Correct answer: Positive predictive value
Predictive values such as PPV depend on disease prevalence, whereas sensitivity, specificity, and likelihood ratios are intrinsic properties of the test and do not vary with prevalence.
If the variance of a dataset is 16, the standard deviation is:
- A 2
- B 4
- C 8
- D 16
Correct answer: 4
Standard deviation is the square root of variance: √16 = 4.
A researcher wants to reduce the margin of error in an estimate. Which change helps achieve this?
- A Decrease sample size
- B Increase sample size
- C Switch from mean to median
- D Decrease confidence interval to 50%
Correct answer: Increase sample size
Larger samples produce more precise estimates with smaller margins of error.
A screening test has a sensitivity of 95% and specificity of 20%. Which consequence is likely?
- A Many false positives
- B Many false negatives
- C High PPV
- D High NPV
Correct answer: Many false positives
Low specificity results in many false positives despite high sensitivity.
In a normally distributed dataset, approximately what proportion lies within 2 standard deviations of the mean?
- A 50%
- B 68%
- C 95%
- D 99%
Correct answer: 95%
In a normal distribution, 95% of values lie within ±2 SD of the mean.
A test’s negative predictive value increases when:
- A Prevalence increases
- B Prevalence decreases
- C Specificity decreases
- D Sensitivity decreases
Correct answer: Prevalence decreases
NPV increases in low-prevalence settings because negative results are more likely true negatives.
A researcher compares the mean blood pressure of two independent groups. Which statistical test is most appropriate?
- A Chi-square test of independence
- B Paired samples t-test
- C Independent samples t-test
- D One-way ANOVA
Correct answer: Independent samples t-test
An independent samples t-test compares means between two unrelated groups.
Relative risk is most appropriately calculated in which study design?
- A Case-control study
- B Cross-sectional study
- C Cohort study
- D Ecologic study
Correct answer: Cohort study
Cohort studies follow exposed and unexposed individuals to compare incidence and calculate relative risk.