Biostatistics Practice Questions

Q: In a vaccine trial, relative risk is 0.2. What is the vaccine efficacy?

80%. ***80%*** - Vaccine efficacy is calculated as **(1 - Relative Risk) x 100%**. Given a relative risk of 0.2, the efficacy is (1 - 0.2) x 100% = **80%**. - This value represents the **proportionate reduction** in disease incidence in the vaccinated group compared to an unvaccinated group. *90%* - This would imply a relative risk of 0.1, as **(1 - 0.1) x 100% = 90%**. - The given relative risk of **0.2** does not correspond to 90% efficacy. *95%* - This would imply a relative risk of 0.05, as **(1 - 0.05) x 100% = 95%**. - The given relative risk of **0.2** does not correspond to 95% efficacy. *20%* - This value directly represents the **Relative Risk (RR)** itself, or an efficacy calculated incorrectly as RR x 100%. - Vaccine efficacy is a measure of reduction from the unvaccinated state, hence it is **1 - RR**.

Q: Hardy-Weinberg equilibrium shows gene frequency p=0.7, q=0.3. What is frequency of heterozygotes?

0.42. ***0.42*** - In **Hardy-Weinberg equilibrium**, the frequency of heterozygotes is given by the formula **2pq**. - Given **p = 0.7** and **q = 0.3**, the frequency of heterozygotes is 2 * 0.7 * 0.3 = **0.42**. *0.09* - This value represents **q²**, which is the frequency of the **homozygous recessive genotype** (0.3 * 0.3 = 0.09). - It does not represent the frequency of heterozygous individuals. *0.49* - This value represents **p²**, which is the frequency of the **homozygous dominant genotype** (0.7 * 0.7 = 0.49). - It does not represent the frequency of heterozygous individuals. *0.21* - This value represents only **pq** (0.7 * 0.3 = 0.21), not the full frequency of heterozygotes which is **2pq**. - The coefficient of 2 is necessary because there are two ways to be heterozygous (one allele from each parent).

Q: Calculate the sensitivity of a screening test: True Positives=80, False Negatives=20, True Negatives=90, False Positives=10

80%. ***80%*** - Sensitivity is calculated as **True Positives / (True Positives + False Negatives)**. In this case, 80 / (80 + 20) = 80/100, which equals 0.8 or 80%. - This metric represents the proportion of **actual positive cases** that are correctly identified by the test. *90%* - This value might represent the **specificity** (True Negatives / (True Negatives + False Positives)) if calculated with the given numbers (90 / (90 + 10) = 90%). - However, the question specifically asks for **sensitivity**, which is a different measure. *85%* - This percentage would be obtained if the total number of true positives and false negatives was 94 (e.g., 80 / 94), which is not the case here. - It does not correspond to the correct formula for **sensitivity** using the provided data. *95%* - This result would occur if the test correctly identified 95 out of 100 actual positive cases (e.g., 95 TP and 5 FN). - The given data of **80 True Positives** and **20 False Negatives** leads to a lower sensitivity.

Q: What is the correlation coefficient if regression coefficient of X on Y is 0.8 and Y on X is 0.9?

0.85. ***Correct: 0.85*** - The correlation coefficient (r) is the **geometric mean** of the two regression coefficients - Formula: r = √(b_xy × b_yx), where b_xy is the regression coefficient of X on Y and b_yx is the regression coefficient of Y on X - Calculation: r = √(0.8 × 0.9) = √0.72 ≈ **0.8485**, which rounds to **0.85** - Since both regression coefficients are positive, the correlation is positive *Incorrect: 0.95* - This would be obtained by taking the **arithmetic mean** [(0.8 + 0.9)/2 = 0.85... wait, that's not 0.95] - Actually, this value is too high and doesn't result from any standard calculation with these regression coefficients - The correct method requires the **geometric mean** (square root of the product), not any simple average *Incorrect: 0.81* - This appears to be the square of one regression coefficient (0.9² = 0.81) - However, the correlation coefficient requires the **square root of the product** of both coefficients, not squaring a single coefficient - This is a common error in calculation *Incorrect: 0.72* - This is the **product** of the two regression coefficients (0.8 × 0.9 = 0.72) - This is an intermediate step in the calculation, but not the final answer - The correlation coefficient requires taking the **square root** of this product: √0.72 ≈ 0.85

Q: The crude birth rate for a sub-center with a population of 1000 is 20. What is the estimated number of pregnant women registered in the sub-center during the year?

60. ***60*** - The **crude birth rate** of 20 means 20 live births per 1,000 population. For a population of 1,000 people, this means there were **20 live births** in that year. - Using the standard epidemiological formula: **Estimated pregnancies = Live births × 3** - This multiplier of 3 accounts for stillbirths, abortions, miscarriages, and ongoing pregnancies that are registered but may not result in live births. - Therefore: **20 live births × 3 = 60 registered pregnant women** ✓ *110* - This value would require a multiplier of 5.5 (110/20), which is significantly higher than the standard epidemiological estimation. - It overestimates the pregnancy-to-live-birth ratio and does not align with established public health calculations. *80* - This implies a multiplier of 4 (80/20), which exceeds the standard ratio of 3 used in community medicine. - While some regions may have higher pregnancy wastage, this is not the standard calculation method. *100* - This suggests a multiplier of 5 (100/20), which greatly overestimates registered pregnancies relative to live births. - This does not correspond to the accepted formula used in Indian public health programs and NEET PG examinations.

Q: What is the sensitivity of a screening test that correctly identifies 90 out of 100 true positive cases?

90%. **90%** - **Sensitivity** is calculated as the number of **true positives** divided by the sum of true positives and **false negatives**. - In this case, 90 true positive cases out of 100 total true cases (90 true positives + 10 false negatives) equals 90/100, or **90%**. *100%* - A sensitivity of **100%** would mean that the test correctly identified all 100 true positive cases. - This value indicates that there were **no false negatives**, which is not the case here. *80%* - An **80%** sensitivity would mean that only 80 out of 100 true positive cases were correctly identified. - This would imply **20 false negatives**, which is less accurate than the given scenario. *85%* - An **85%** sensitivity would mean that 85 out of 100 true positive cases were correctly identified. - This implies **15 false negatives**, which is also less accurate than the given scenario of 90 true positives.

Q: Bias in clinical trials can be minimized by all of the following methods, EXCEPT:

Multivariate analysis. ***Multivariate analysis*** - **Multivariate analysis** is a statistical tool used to analyze data with multiple variables, but it cannot prevent bias during the design or execution phases of a clinical trial. - While it can help control for confounding variables during data analysis, it is applied *after* data collection and does not eliminate sources of bias related to participant selection, intervention assignment, or outcome assessment. *Matching* - **Matching** involves selecting control group participants who share similar characteristics (e.g., age, sex, comorbidities) with the intervention group, thereby reducing confounding by known variables. - This method helps to ensure that groups are comparable at baseline, minimizing **selection bias** and making observed differences more attributable to the intervention. *Blinding* - **Blinding** involves concealing the treatment assignment from participants, researchers, or both (single- or double-blinding) to prevent their expectations or preconceptions from influencing outcomes or assessments. - This method effectively minimizes various forms of **performance bias** (e.g., in participant behavior or co-interventions) and **detection bias** (e.g., in outcome assessment). *Randomization* - **Randomization** is the process of assigning participants to intervention or control groups purely by chance, which helps ensure that known and unknown confounding factors are evenly distributed between groups. - This method is crucial for minimizing **selection bias**, making the groups comparable and increasing the likelihood that any observed differences are due to the intervention rather than pre-existing differences.

Q: A study finds that a new diet reduces the risk of heart disease by 30%. What type of measure does this represent?

Relative risk reduction. ***Relative risk reduction*** - **Relative risk reduction (RRR)** is the percentage reduction in risk in the exposed group compared to the unexposed group. - A 30% reduction in risk specifically indicates RRR, calculated as: **(risk in unexposed - risk in exposed) / risk in unexposed × 100%**. *Absolute risk reduction* - **Absolute risk reduction (ARR)** is the difference in risk between the exposed and unexposed groups. - It is expressed as a **percentage point difference**, not a percentage *reduction* of the original risk. *Odds ratio* - The **odds ratio (OR)** quantifies the odds of an event occurring in one group compared to the odds of it occurring in another group. - It is typically used in **case-control studies** and does not directly express a reduction in risk. *Number needed to treat* - **Number needed to treat (NNT)** is the number of patients who need to be treated to prevent one additional adverse outcome. - It is calculated as the **reciprocal of the absolute risk reduction (1/ARR)**.

Question 1

In a vaccine trial, relative risk is 0.2. What is the vaccine efficacy?

Accepted Answer

80%

Answer

90%

Answer

95%

Answer

20%

Question 2

Hardy-Weinberg equilibrium shows gene frequency p=0.7, q=0.3. What is frequency of heterozygotes?

Accepted Answer

0.42

Answer

0.09

Answer

0.49

Answer

0.21

Question 3

Calculate the sensitivity of a screening test: True Positives=80, False Negatives=20, True Negatives=90, False Positives=10

Accepted Answer

80%

Answer

90%

Answer

85%

Answer

95%

Question 4

What is the correlation coefficient if regression coefficient of X on Y is 0.8 and Y on X is 0.9?

Accepted Answer

0.85

Answer

0.95

Answer

0.81

Answer

0.72

Question 5

The crude birth rate for a sub-center with a population of 1000 is 20. What is the estimated number of pregnant women registered in the sub-center during the year?

Accepted Answer

60

Answer

110

Answer

80

Answer

100

Question 6

What is the sensitivity of a screening test that correctly identifies 90 out of 100 true positive cases?

Accepted Answer

90%

Answer

100%

Answer

80%

Answer

85%

Question 7

A study comparing two antihypertensive drugs reports a p-value of 0.03 for the primary outcome. What is the correct interpretation of this result?

Accepted Answer

There is a 3% chance that the observed difference occurred by chance if the null hypothesis is true.

Answer

The probability of making a Type I error is 3%.

Answer

The study has a 97% chance of detecting a true difference if it exists.

Answer

There is a 3% chance that the null hypothesis is false.

Question 8

Bias in clinical trials can be minimized by all of the following methods, EXCEPT:

Accepted Answer

Multivariate analysis

Answer

Matching

Answer

Randomization

Answer

Blinding

Question 9

A researcher is conducting a study to compare two treatment modalities for hypertension. The study finds a p-value of 0.02 and a 95% confidence interval for the difference in means that does not include zero. How should these results be interpreted?

Accepted Answer

The results are statistically significant, indicating that the null hypothesis can be rejected.

Answer

There is no significant difference between the two treatments.

Answer

The p-value indicates that the results are not significant.

Answer

The confidence interval suggests that the study has low power.

Question 10

A study finds that a new diet reduces the risk of heart disease by 30%. What type of measure does this represent?

Accepted Answer

Relative risk reduction

Answer

Absolute risk reduction

Answer

Odds ratio

Answer

Number needed to treat

Biostatistics — MCQs

Biostatistics — MCQs

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