Epidemiology — MCQs
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A screening test has sensitivity of 90% and specificity of 99%. The prevalence of disease under investigation is 5 per 1000 population. What is the PPV of the given screening test?
Which of the following statements is true about cohort studies?
Which of the following correctly describes the components of the epidemiological triad?
Best indicator for spread of TB in a community?
Strength of association of outcome and risk factor is measured by?
Which measure is considered crucial for formulating a National Health Policy?
Sensitivity of a screening test tells about
What is the significance of the 1st of July in demographic studies?
In a class of 100 students, 40 are susceptible to chickenpox. If 28 students developed chickenpox within the next 2 weeks, what is the secondary attack rate (SAR) of chickenpox?
During investigation of an epidemic, the area is declared free of epidemic when?
Epidemiology Indian Medical PG Practice Questions and MCQs
Question 1711: A screening test has sensitivity of 90% and specificity of 99%. The prevalence of disease under investigation is 5 per 1000 population. What is the PPV of the given screening test?
- A. 10
- B. 70
- C. 33 (Correct Answer)
- D. 99
Explanation: ***33*** * **Positive Predictive Value (PPV)** = \[Sensitivity \times Prevalence] / \[(Sensitivity \times Prevalence) + (1 – Specificity) \times (1 – Prevalence)] * Here, 0.9 x 0.005 / \[(0.9 x 0.005) + (1-0.99) x (1-0.005)] = 0.0045 / \[0.0045 + (0.01 x 0.995)] = 0.0045 / \[0.0045 + 0.00995] = 0.0045 / 0.01445 ≈ 0.3114 or 31.14%. Converting to a percentage closest to the answer choices, it would be 33%. *10* * This value would be obtained if there was an error in calculating either the **prevalence** of the disease or the contribution of false positives to the total positive tests. * Inaccuracies in the formula or arithmetic would lead to results far from the correct PPV. *70* * This value suggests a much higher **prevalence** or a significantly lower number of **false positives** than indicated by the given sensitivity, specificity, and prevalence. * Such a high PPV is inconsistent with a low disease prevalence (0.5%) and a relatively high false positive rate (1-specificity = 1%). *99* * This value is close to the given **specificity**, which is the probability of a true negative test among individuals without the disease, not the PPV. * A PPV of 99% would be extremely high for a disease with a prevalence of 0.5% and would typically require a much higher specificity and/or sensitivity.
Question 1712: Which of the following statements is true about cohort studies?
- A. A study that follows participants over time to observe outcomes but does not measure incidence.
- B. A study that describes characteristics of a population.
- C. A study that can determine cause and effect.
- D. A study that measures the incidence of a disease. (Correct Answer)
Explanation: ***A study that measures the incidence of a disease.*** - Cohort studies are **prospective studies** that follow a defined group of individuals over time to observe the development of diseases or health outcomes [1]. - By following an at-risk population and documenting new cases, they can directly calculate the **incidence rate** of a disease [1]. - This is the **primary strength** of cohort studies - they provide the best epidemiological evidence for disease incidence. *A study that describes characteristics of a population.* - This describes **descriptive studies** or **cross-sectional surveys**, which characterize a population at a single point in time. - While cohort studies may initially describe baseline characteristics, their primary purpose is to observe disease occurrence over time, not just description. *A study that follows participants over time to observe outcomes but does not measure incidence.* - This is **contradictory** - the act of following participants over time and observing new disease cases IS the measurement of incidence [1]. - Incidence (new cases per unit of person-time) is precisely what cohort studies are designed to measure. *A study that can determine cause and effect.* - While cohort studies establish **temporal relationships** (exposure precedes outcome) and provide strong evidence for causality, the word "determine" is too absolute. - Establishing definitive causation requires **multiple lines of evidence**, including criteria like biological plausibility, dose-response relationships, and consistency across studies. - **Randomized controlled trials** provide stronger causal evidence due to randomization eliminating confounding.
Question 1713: Which of the following correctly describes the components of the epidemiological triad?
- A. Agent, host, environment (Correct Answer)
- B. Agent, disease, vector
- C. Time, place, person
- D. Disease, prevention, treatment
Explanation: **Agent, host, environment** * The **epidemiological triad** is a classic model that describes the interrelationships contributing to disease development. * It consists of the **agent** (the causative factor), the **host** (the organism contracting the disease), and the **environment** (extrinsic factors influencing exposure and susceptibility). * *Time, place, person (descriptive epidemiology)* * **Time, place, and person** are key components of **descriptive epidemiology**, used to characterize disease patterns. * While important for understanding disease distribution, they do not represent the causative interconnectedness of the epidemiological triad. * *Disease, prevention, treatment (healthcare process)* * **Disease, prevention, and treatment** describe aspects of the healthcare process and public health interventions. * They are not the fundamental components used to explain the *occurrence* of disease as modeled by the epidemiological triad. * *Agent, disease, vector (lacks host and environmental components)* * This option includes the **agent** and a **vector** (which is a type of environmental factor or can be considered part of the agent's transmission). * However, it misses the crucial component of the **host** and fully encompassing the broader **environment**.
Question 1714: Best indicator for spread of TB in a community?
- A. Prevalence of infection
- B. Annual infection rate (Correct Answer)
- C. Incidence of new cases
- D. Case rate
Explanation: ***Annual infection rate*** - The **Annual Infection Rate (ARI)** is the gold standard indicator for measuring TB spread in a community - It measures the rate at which new TB infections occur in a tuberculin-negative population over one year - ARI directly reflects **recent transmission** and the force of infection in the community - WHO and Park's Textbook recommend ARI as the best epidemiological indicator for TB transmission - Calculated through tuberculin surveys showing conversion from negative to positive - An ARI of 1% indicates 1% of uninfected individuals become infected per year *Incidence of new cases* - Incidence measures new **disease cases** (active TB), not new infections - Only 5-10% of TB infections progress to active disease, often after years of latency - Incidence is affected by case detection rates, diagnostic capacity, and healthcare access - It underestimates actual transmission occurring in the community *Prevalence of infection* - **Prevalence** indicates total existing cases (both old and new) at a point in time - Influenced by both new infections and duration of infection/disease - Does not specifically measure the rate of ongoing transmission *Case rate* - **Case rate** refers to the number of active disease cases per population - Similar to prevalence, it doesn't isolate new transmission events - Less sensitive for detecting changes in transmission dynamics
Question 1715: Strength of association of outcome and risk factor is measured by?
- A. Relative risk (Correct Answer)
- B. Attributable risk
- C. Population attributable risk
- D. None of the options
Explanation: ***Relative risk*** - **Relative risk (RR)** directly measures the **strength of association** between exposure to a risk factor and the development of an outcome. - It is calculated as the ratio of the **incidence rate** in the exposed group to the incidence rate in the unexposed group. *Attributable risk* - **Attributable risk (AR)** measures the **absolute difference** in outcome rates between exposed and unexposed groups, indicating the extra cases due to exposure. - It quantifies the **public health impact** or burden of disease that can be attributed to the exposure, rather than the strength of association. *Population attributable risk* - **Population attributable risk (PAR)** estimates the proportion of disease incidence in the **total population** that is attributable to an exposure. - It considers both the **strength of the association** and the **prevalence of the exposure** in the population, making it more about public health impact than individual strength of association. *None of the options* - This option is incorrect because **relative risk** is a fundamental epidemiological measure for determining the strength of association.
Question 1716: Which measure is considered crucial for formulating a National Health Policy?
- A. Measure of association between exposure and outcome (Relative risk)
- B. Frequency of new disease cases (Incidence rate)
- C. Proportion of cases linked to exposure (Attributable risk) (Correct Answer)
- D. Estimate of exposure-outcome odds (Odds ratio)
Explanation: ***Proportion of cases linked to exposure (Attributable risk)*** - **Attributable risk** quantifies the proportion of disease cases in a population that can be attributed to specific **exposures**. - This measure is crucial for health policy as it helps prioritize interventions by identifying diseases and their causative factors that, if eliminated, would lead to the largest reduction in disease burden. *Measure of association between exposure and outcome (Relative risk)* - **Relative risk** indicates the strength of the association between an exposure and an outcome, comparing the risk of disease in exposed versus unexposed groups. - While important for understanding etiology, it doesn't directly quantify the **health burden** in the population that could be prevented by removing the exposure. *Frequency of new disease cases (Incidence rate)* - **Incidence rate** measures the rate at which new cases of a disease occur in a population over a specified period. - While it provides insight into disease spread, it doesn't directly identify how much of that spread is **preventable** by addressing specific risk factors for policy formulation. *Estimate of exposure-outcome odds (Odds ratio)* - The **odds ratio** is an estimate of the likelihood of an outcome occurring given exposure, usually in case-control studies. - Similar to relative risk, it indicates the **strength of association** but doesn't directly translate into the preventable disease burden at a population level.
Question 1717: Sensitivity of a screening test tells about
- A. Percentage of individuals with the disease who test positive. (Correct Answer)
- B. Percentage of healthy individuals among those with a negative test result.
- C. Percentage of individuals with the disease among those with a positive test result.
- D. Percentage of healthy individuals among those with a positive test result.
Explanation: ***Percentage of individuals with the disease who test positive*** - **Sensitivity** measures the ability of a test to correctly identify individuals who *have* the **disease**. - It's calculated as (True Positives / (True Positives + False Negatives)) * 100, representing the **true positive rate**. *Percentage of healthy individuals among those with a negative test result* - This describes the **negative predictive value (NPV)**, which is the probability that a person who tests negative truly does not have the disease. - NPV is crucial for ruling out disease in a population. *Percentage of individuals with the disease among those with a positive test result* - This is the definition of **positive predictive value (PPV)**, indicating the probability that a person who tests positive truly has the disease. - PPV is important for confirming a diagnosis in clinical practice. *Percentage of healthy individuals among those with a positive test result* - This describes 1 minus the positive predictive value, or the rate of **false positives** among those who test positive. - A high rate here means many healthy individuals are incorrectly identified as having the disease.
Question 1718: What is the significance of the 1st of July in demographic studies?
- A. 1st January
- B. 1st July (Correct Answer)
- C. 1st September
- D. 15th June
Explanation: ***1st July*** - The **1st of July** is often considered the **mid-year population estimate** in demographic studies. - This date is crucial for calculating rates (e.g., birth rates, death rates) and making projections, as it provides a standardized reference point that balances out seasonal population fluctuations. *1st January* - While a significant date for many annual administrative purposes, the **1st of January** represents the beginning of the year, not the mid-point. - Using this date for population counts can sometimes overemphasize the population at the start of a new calendar cycle, potentially skewing annual rate calculations. *1st September* - The **1st of September** falls in the third quarter of the year and does not represent a standard mid-year point for demographic estimations. - This date has no widely recognized statistical significance for population estimation in demographic contexts. *15th June* - The **15th of June** is close to the middle of the year but is not the conventionally adopted date for mid-year population estimates. - Demographers prefer standard, easily replicable dates for consistency across regions and studies.
Question 1719: In a class of 100 students, 40 are susceptible to chickenpox. If 28 students developed chickenpox within the next 2 weeks, what is the secondary attack rate (SAR) of chickenpox?
- A. 60%
- B. 90%
- C. 80%
- D. 70% (Correct Answer)
Explanation: **70% - Correct Answer** The **secondary attack rate (SAR)** measures the spread of disease among susceptible contacts after exposure to a primary case. It is calculated as: **SAR = (Number of new cases among susceptible contacts / Total number of susceptible contacts) × 100%** In this scenario: - New cases = 28 students - Susceptible contacts = 40 students - SAR = (28/40) × 100% = **70%** This is the correct application of the SAR formula, using only susceptible individuals as the denominator (not the total class of 100). *60% - Incorrect* - This would require 24 out of 40 susceptible students developing chickenpox (24/40 = 0.60) - Does not match the observed 28 cases *80% - Incorrect* - This would require 32 out of 40 susceptible students developing chickenpox (32/40 = 0.80) - Overestimates the number of cases compared to the observed 28 *90% - Incorrect* - This would require 36 out of 40 susceptible students developing chickenpox (36/40 = 0.90) - Significantly overestimates the attack rate and does not reflect the actual 28 cases observed
Question 1720: During investigation of an epidemic, the area is declared free of epidemic when?
- A. Twice the incubation period of the disease since occurrence of the last case (Correct Answer)
- B. Thrice the incubation period of the disease since occurrence of the last case
- C. The longest incubation period for the disease
- D. Incubation period for the disease plus two standard deviations
Explanation: ***Twice the incubation period of the disease since occurrence of the last case*** - An epidemic is declared over when there have been no new cases for a period equal to **twice the maximum incubation period** of the disease. - This timeframe ensures that any individuals who might have been infected by the last known case would have developed symptoms (or completed their infectivity period) within this observation window. *Thrice the incubation period of the disease since occurrence of the last case* - This duration is **excessively long** and not the standard public health measure for declaring an epidemic free. - While it provides an even greater margin of safety, it is not the most **efficient or practical threshold** for public health interventions. *The longest incubation period for the disease* - Observing for only the **longest incubation period** is insufficient because a new case could still emerge at the very end of this period, potentially starting a new chain of transmission. - It does not account for the possibility of **secondary cases** occurring at the extreme end of the incubation period. *Incubation period for the disease plus two standard deviations* - This statistical measure is typically used to define the **range of normal variation** for biological data, not for epidemic declaration. - While it relates to the incubation period, it is **not the established public health standard** for determining the end of an epidemic.
Practice by Chapter
Principles of Epidemiology
Practice Questions
Measures of Disease Frequency
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Epidemiological Study Designs
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Descriptive Epidemiology
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Analytical Epidemiology
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Experimental Epidemiology
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Screening for Disease
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Surveillance Systems
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Investigation of an Epidemic
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Association and Causation
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Modern Epidemiological Methods
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Critical Appraisal of Epidemiological Studies
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