PUBH210 Epidemiology Exam Prep – Real Practice Questions, Answers & Detailed
Rationales (Updated 2026) | Disease Transmission & Outbreak
Investigation, Incidence & Prevalence Rates, Study Designs (Cohort, Case-Control,
Cross-Sectional), Public Health Surveillance, Risk Factors & Bias, Screening &
Prevention Strategies, Biostatistics Basics, Population Health & Data
Interpretation
Question 1: Which measure of disease frequency represents the proportion of a
population that has a condition at a specific point in time?
A. Incidence rate
B. Cumulative incidence
C. Period prevalence
D. Point prevalence
CORRECT ANSWER: D. Point prevalence
Rationale: Point prevalence quantifies the proportion of individuals in a population who
have a disease or condition at a single, specified point in time. It is calculated as
(number of existing cases at a point in time) / (total population at that time). Incidence
measures new cases, while period prevalence covers a time interval.
Question 2: In a cohort study investigating the association between smoking and
lung cancer, researchers follow 10,000 smokers and 10,000 non-smokers for 20
years. Which measure of association is most appropriate to calculate?
A. Odds ratio
B. Prevalence ratio
C. Relative risk
D. Attributable risk percent
CORRECT ANSWER: C. Relative risk
Rationale: Cohort studies follow exposed and unexposed groups forward in time to
observe disease incidence, allowing direct calculation of relative risk (RR = incidence in
exposed / incidence in unexposed). Odds ratios are typically used in case-control
studies where incidence cannot be directly measured.
Question 3: A screening test for diabetes has a sensitivity of 90% and a specificity
of 85%. If the prevalence of diabetes in the population is 10%, what is the
approximate positive predictive value?
A. 40%
B. 60%
C. 75%
D. 90%
CORRECT ANSWER: B. 60%
Rationale: Using Bayes' theorem or a 2x2 table with 1000 people: 100 have diabetes
(10% prevalence). True positives = 90 (90% of 100), false positives = 135 (15% of 900
,non-diabetics). PPV = TP/(TP+FP) = 90/(90+135) = 90/225 = 40%. Wait, let me
recalculate: 90/225 = 0.40 = 40%. Actually the correct answer should be A. Let me
verify: With prevalence 10%, sensitivity 90%, specificity 85%: PPV = (sens × prev) /
[(sens × prev) + ((1-spec) × (1-prev))] = (0.9×0.1) / [(0.9×0.1) + (0.15×0.9)] = 0.09 / (0.09 +
0.135) = 0.09/0.225 = 0.40 = 40%. So the correct answer is A. 40%.
CORRECT ANSWER: A. 40%
Rationale: Positive predictive value depends on prevalence, sensitivity, and specificity.
Using the formula PPV = (sensitivity × prevalence) / [(sensitivity × prevalence) + ((1 −
specificity) × (1 − prevalence))], we calculate: (0.90 × 0.10) / [(0.90 × 0.10) + (0.15 ×
0.90)] = 0..225 = 0.40 or 40%. This demonstrates that even with good test
characteristics, low prevalence reduces PPV.
Question 4: Which type of bias occurs when the association between exposure and
outcome differs between those who participate in a study and those who do not?
A. Recall bias
B. Selection bias
C. Information bias
D. Confounding bias
CORRECT ANSWER: B. Selection bias
Rationale: Selection bias arises when the relationship between exposure and disease
differs for those who participate versus those who are eligible but do not participate.
This can distort effect estimates if participation is related to both exposure and
outcome. Recall bias is a type of information bias related to differential memory of past
exposures.
Question 5: The basic reproduction number (R₀) of an infectious disease
represents:
A. The average number of secondary cases generated by one primary case in a fully
susceptible population
B. The proportion of the population that must be immune to achieve herd immunity
C. The case fatality rate during an epidemic peak
D. The incubation period multiplied by the infectious period
CORRECT ANSWER: A. The average number of secondary cases generated by one
primary case in a fully susceptible population
Rationale: R₀ is a fundamental metric in infectious disease epidemiology that
quantifies transmissibility. It represents the expected number of secondary infections
produced by a single infected individual in a completely susceptible population. Herd
immunity threshold is calculated as 1 − 1/R₀, not R₀ itself.
Question 6: In a case-control study, the odds ratio is used to estimate the relative
risk when:
,A. The disease is common (>10% prevalence)
B. The disease is rare (<10% prevalence)
C. Exposure is rare
D. The study uses matching
CORRECT ANSWER: B. The disease is rare (<10% prevalence)
Rationale: The rare disease assumption allows the odds ratio from a case-control study
to approximate the relative risk. When disease incidence is low, the number of non-
cases approximates the total population, making the odds of exposure among cases
and controls closely estimate the risk ratio. This approximation breaks down when
disease is common.
Question 7: Which Bradford Hill criterion refers to the observation that the
association is consistent across different studies, populations, and
methodologies?
A. Strength of association
B. Consistency
C. Specificity
D. Coherence
CORRECT ANSWER: B. Consistency
Rationale: Consistency is one of Bradford Hill's viewpoints for causal inference,
indicating that an association observed repeatedly by different persons, in different
places, circumstances, and times strengthens the evidence for causality. It does not
prove causation but reduces the likelihood that findings are due to chance or bias
specific to one study.
Question 8: A public health department identifies 50 cases of salmonellosis in a
week when the expected number is 5. This situation is best described as:
A. Endemic
B. Epidemic
C. Pandemic
D. Sporadic
CORRECT ANSWER: B. Epidemic
Rationale: An epidemic is defined as the occurrence of cases of a disease in excess of
what would normally be expected in a defined community, geographical area, or
season. Here, 50 cases versus an expected 5 represents a clear excess. Endemic refers
to constant presence, pandemic to worldwide spread, and sporadic to occasional,
irregular occurrence.
Question 9: Which study design is most appropriate for investigating the incidence
of a rare disease?
, A. Cross-sectional study
B. Case-control study
C. Cohort study
D. Ecological study
CORRECT ANSWER: B. Case-control study
Rationale: Case-control studies are efficient for rare diseases because they start with
identified cases and select controls, avoiding the need to follow a large cohort for a long
time to accrue sufficient cases. Cohort studies are inefficient for rare outcomes due to
the large sample sizes required. Cross-sectional studies measure prevalence, not
incidence.
Question 10: The attributable risk percent in the exposed group is calculated as:
A. (RR − 1) / RR × 100%
B. (Incidence exposed − Incidence unexposed) / Incidence exposed × 100%
C. Both A and B
D. (Incidence exposed − Incidence unexposed) / Incidence unexposed × 100%
CORRECT ANSWER: C. Both A and B
Rationale: Attributable risk percent (AR%) in the exposed can be expressed two
equivalent ways: (1) using incidence rates: (Ie − Iu)/Ie × 100%, or (2) using relative risk:
(RR − 1)/RR × 100%, since RR = Ie/Iu. Both formulas estimate the proportion of disease
among the exposed that is attributable to the exposure, assuming causality.
Question 11: Which measure describes the time from infection to the onset of
clinical symptoms?
A. Serial interval
B. Generation time
C. Incubation period
D. Latent period
CORRECT ANSWER: C. Incubation period
Rationale: The incubation period is the interval between infection (or exposure) and the
appearance of clinical signs or symptoms. The latent period refers to time from
infection to infectiousness. Generation time is the interval between infection in a
primary case and infection in a secondary case. Serial interval is the time between
symptom onset in primary and secondary cases.
Question 12: In a randomized controlled trial, the primary purpose of
randomization is to:
A. Ensure the study population is representative of the general population
B. Minimize selection bias in assignment to intervention groups
C. Guarantee that the treatment and control groups are identical in all respects
D. Eliminate the need for blinding
Rationales (Updated 2026) | Disease Transmission & Outbreak
Investigation, Incidence & Prevalence Rates, Study Designs (Cohort, Case-Control,
Cross-Sectional), Public Health Surveillance, Risk Factors & Bias, Screening &
Prevention Strategies, Biostatistics Basics, Population Health & Data
Interpretation
Question 1: Which measure of disease frequency represents the proportion of a
population that has a condition at a specific point in time?
A. Incidence rate
B. Cumulative incidence
C. Period prevalence
D. Point prevalence
CORRECT ANSWER: D. Point prevalence
Rationale: Point prevalence quantifies the proportion of individuals in a population who
have a disease or condition at a single, specified point in time. It is calculated as
(number of existing cases at a point in time) / (total population at that time). Incidence
measures new cases, while period prevalence covers a time interval.
Question 2: In a cohort study investigating the association between smoking and
lung cancer, researchers follow 10,000 smokers and 10,000 non-smokers for 20
years. Which measure of association is most appropriate to calculate?
A. Odds ratio
B. Prevalence ratio
C. Relative risk
D. Attributable risk percent
CORRECT ANSWER: C. Relative risk
Rationale: Cohort studies follow exposed and unexposed groups forward in time to
observe disease incidence, allowing direct calculation of relative risk (RR = incidence in
exposed / incidence in unexposed). Odds ratios are typically used in case-control
studies where incidence cannot be directly measured.
Question 3: A screening test for diabetes has a sensitivity of 90% and a specificity
of 85%. If the prevalence of diabetes in the population is 10%, what is the
approximate positive predictive value?
A. 40%
B. 60%
C. 75%
D. 90%
CORRECT ANSWER: B. 60%
Rationale: Using Bayes' theorem or a 2x2 table with 1000 people: 100 have diabetes
(10% prevalence). True positives = 90 (90% of 100), false positives = 135 (15% of 900
,non-diabetics). PPV = TP/(TP+FP) = 90/(90+135) = 90/225 = 40%. Wait, let me
recalculate: 90/225 = 0.40 = 40%. Actually the correct answer should be A. Let me
verify: With prevalence 10%, sensitivity 90%, specificity 85%: PPV = (sens × prev) /
[(sens × prev) + ((1-spec) × (1-prev))] = (0.9×0.1) / [(0.9×0.1) + (0.15×0.9)] = 0.09 / (0.09 +
0.135) = 0.09/0.225 = 0.40 = 40%. So the correct answer is A. 40%.
CORRECT ANSWER: A. 40%
Rationale: Positive predictive value depends on prevalence, sensitivity, and specificity.
Using the formula PPV = (sensitivity × prevalence) / [(sensitivity × prevalence) + ((1 −
specificity) × (1 − prevalence))], we calculate: (0.90 × 0.10) / [(0.90 × 0.10) + (0.15 ×
0.90)] = 0..225 = 0.40 or 40%. This demonstrates that even with good test
characteristics, low prevalence reduces PPV.
Question 4: Which type of bias occurs when the association between exposure and
outcome differs between those who participate in a study and those who do not?
A. Recall bias
B. Selection bias
C. Information bias
D. Confounding bias
CORRECT ANSWER: B. Selection bias
Rationale: Selection bias arises when the relationship between exposure and disease
differs for those who participate versus those who are eligible but do not participate.
This can distort effect estimates if participation is related to both exposure and
outcome. Recall bias is a type of information bias related to differential memory of past
exposures.
Question 5: The basic reproduction number (R₀) of an infectious disease
represents:
A. The average number of secondary cases generated by one primary case in a fully
susceptible population
B. The proportion of the population that must be immune to achieve herd immunity
C. The case fatality rate during an epidemic peak
D. The incubation period multiplied by the infectious period
CORRECT ANSWER: A. The average number of secondary cases generated by one
primary case in a fully susceptible population
Rationale: R₀ is a fundamental metric in infectious disease epidemiology that
quantifies transmissibility. It represents the expected number of secondary infections
produced by a single infected individual in a completely susceptible population. Herd
immunity threshold is calculated as 1 − 1/R₀, not R₀ itself.
Question 6: In a case-control study, the odds ratio is used to estimate the relative
risk when:
,A. The disease is common (>10% prevalence)
B. The disease is rare (<10% prevalence)
C. Exposure is rare
D. The study uses matching
CORRECT ANSWER: B. The disease is rare (<10% prevalence)
Rationale: The rare disease assumption allows the odds ratio from a case-control study
to approximate the relative risk. When disease incidence is low, the number of non-
cases approximates the total population, making the odds of exposure among cases
and controls closely estimate the risk ratio. This approximation breaks down when
disease is common.
Question 7: Which Bradford Hill criterion refers to the observation that the
association is consistent across different studies, populations, and
methodologies?
A. Strength of association
B. Consistency
C. Specificity
D. Coherence
CORRECT ANSWER: B. Consistency
Rationale: Consistency is one of Bradford Hill's viewpoints for causal inference,
indicating that an association observed repeatedly by different persons, in different
places, circumstances, and times strengthens the evidence for causality. It does not
prove causation but reduces the likelihood that findings are due to chance or bias
specific to one study.
Question 8: A public health department identifies 50 cases of salmonellosis in a
week when the expected number is 5. This situation is best described as:
A. Endemic
B. Epidemic
C. Pandemic
D. Sporadic
CORRECT ANSWER: B. Epidemic
Rationale: An epidemic is defined as the occurrence of cases of a disease in excess of
what would normally be expected in a defined community, geographical area, or
season. Here, 50 cases versus an expected 5 represents a clear excess. Endemic refers
to constant presence, pandemic to worldwide spread, and sporadic to occasional,
irregular occurrence.
Question 9: Which study design is most appropriate for investigating the incidence
of a rare disease?
, A. Cross-sectional study
B. Case-control study
C. Cohort study
D. Ecological study
CORRECT ANSWER: B. Case-control study
Rationale: Case-control studies are efficient for rare diseases because they start with
identified cases and select controls, avoiding the need to follow a large cohort for a long
time to accrue sufficient cases. Cohort studies are inefficient for rare outcomes due to
the large sample sizes required. Cross-sectional studies measure prevalence, not
incidence.
Question 10: The attributable risk percent in the exposed group is calculated as:
A. (RR − 1) / RR × 100%
B. (Incidence exposed − Incidence unexposed) / Incidence exposed × 100%
C. Both A and B
D. (Incidence exposed − Incidence unexposed) / Incidence unexposed × 100%
CORRECT ANSWER: C. Both A and B
Rationale: Attributable risk percent (AR%) in the exposed can be expressed two
equivalent ways: (1) using incidence rates: (Ie − Iu)/Ie × 100%, or (2) using relative risk:
(RR − 1)/RR × 100%, since RR = Ie/Iu. Both formulas estimate the proportion of disease
among the exposed that is attributable to the exposure, assuming causality.
Question 11: Which measure describes the time from infection to the onset of
clinical symptoms?
A. Serial interval
B. Generation time
C. Incubation period
D. Latent period
CORRECT ANSWER: C. Incubation period
Rationale: The incubation period is the interval between infection (or exposure) and the
appearance of clinical signs or symptoms. The latent period refers to time from
infection to infectiousness. Generation time is the interval between infection in a
primary case and infection in a secondary case. Serial interval is the time between
symptom onset in primary and secondary cases.
Question 12: In a randomized controlled trial, the primary purpose of
randomization is to:
A. Ensure the study population is representative of the general population
B. Minimize selection bias in assignment to intervention groups
C. Guarantee that the treatment and control groups are identical in all respects
D. Eliminate the need for blinding
