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ABPM Public Health practice questions

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Question 1Epidemiology

Which measure of disease frequency describes the proportion of a population that has a condition at a specific point in time?

  1. Case fatality rate
  2. Attack rate
  3. Point prevalence Correct answer.
  4. Incidence rate

Explanation

Prevalence counts existing cases in a population at a defined moment or period, regardless of when disease began. Incidence measures new cases over time, so it is distinct from prevalence.

Question 2Epidemiology

In a cohort study, which quantity is estimated directly by comparing disease risk in exposed and unexposed groups?

  1. Prevalence ratio
  2. Relative risk Correct answer.
  3. Attributable fraction in the population
  4. Odds ratio

Explanation

Cohort studies follow groups forward and measure incidence, so the ratio of risks (relative risk) is computed directly. The odds ratio approximates risk only when disease is rare.

Question 3Epidemiology

An outbreak of gastroenteritis affects 60 of 300 people who attended a wedding reception. What is the attack rate?

  1. 50 percent
  2. 10 percent
  3. 20 percent Correct answer.
  4. 5 percent

Explanation

Attack rate equals the number of cases divided by the number at risk, so 60 divided by 300 equals 0.20, or 20 percent. It expresses the proportion of a defined group that develops illness.

Question 4Epidemiology

Which study design is best suited to investigating a rare disease with a long latency period?

  1. Case-control study Correct answer.
  2. Cross-sectional survey
  3. Randomized controlled trial
  4. Prospective cohort study

Explanation

Case-control studies begin with diseased and non-diseased people and look back at exposures, making them efficient for rare outcomes. Cohort designs would need very large samples and long follow-up to accrue enough cases.

Question 5Epidemiology

A screening test's ability to correctly identify people who truly have the disease is called:

  1. Negative predictive value
  2. Sensitivity Correct answer.
  3. Specificity
  4. Positive predictive value

Explanation

Sensitivity is the proportion of true cases that the test classifies as positive. Specificity concerns true non-cases, while predictive values depend on prevalence.

Question 6Epidemiology

Which term describes the proportion of people with a disease who die from it within a specified period?

  1. Case fatality rate Correct answer.
  2. Attack rate
  3. Mortality rate
  4. Incidence proportion

Explanation

Case fatality rate is deaths among diagnosed cases divided by the number of cases, reflecting disease severity. Mortality rate uses the whole population as the denominator.

Question 7Epidemiology

Which measure of association is calculated as the ratio of the odds of exposure among cases to the odds of exposure among controls?

  1. Hazard ratio
  2. Risk ratio
  3. Rate ratio
  4. Odds ratio Correct answer.

Explanation

The odds ratio compares exposure odds between cases and controls and is the standard measure from case-control studies. Risk and rate ratios require incidence data from a defined cohort.

Question 8Epidemiology

Which of the following is an example of a primary prevention activity?

  1. Annual eye exams for people with diabetes
  2. Mammography screening for breast cancer
  3. Physical therapy after a stroke
  4. Immunization against measles Correct answer.

Explanation

Primary prevention acts before disease occurs by reducing risk in susceptible people, as vaccination does. Screening and rehabilitation act at later stages of the disease process.

Question 9Epidemiology

A test has sensitivity of 90 percent and specificity of 95 percent. In a population with a disease prevalence of 1 percent, which property of the test will be most affected?

  1. Specificity will rise substantially
  2. Sensitivity will fall substantially
  3. Positive predictive value will be low Correct answer.
  4. Negative predictive value will be low

Explanation

When prevalence is very low, most positive results arise from the larger pool of false positives, so positive predictive value falls even with good sensitivity and specificity. Sensitivity and specificity are intrinsic to the test and do not change with prevalence.

Question 10Epidemiology

In a case-control study, 40 of 100 cases and 20 of 200 controls were exposed to a suspected risk factor. What is the odds ratio?

  1. 6.0 Correct answer.
  2. 2.0
  3. 4.0
  4. 3.0

Explanation

Exposure odds among cases is 40/60, and among controls is 20/180. The ratio is (40 times 180) divided by (60 times 20), which equals 6.0.

Question 11Epidemiology

A study reports a relative risk of 2.5 with a 95 percent confidence interval of 1.2 to 5.2. Which interpretation is most appropriate?

  1. The association is statistically significant and suggests increased risk Correct answer.
  2. The confidence interval indicates serious random error in sampling
  3. The result proves that the exposure causes the disease
  4. The association is not statistically significant at the 5 percent level

Explanation

Because the confidence interval excludes 1.0, the association is statistically significant at the 5 percent level and indicates higher risk among the exposed. Statistical significance does not by itself establish causation.

Question 12Epidemiology

Which type of bias occurs when the way participants are selected makes the study groups unrepresentative of the target population?

  1. Confounding
  2. Recall bias
  3. Information bias
  4. Selection bias Correct answer.

Explanation

Selection bias arises from the procedures used to enroll participants, producing groups that differ systematically from the source population. Recall and information bias concern measurement, and confounding concerns a third variable.

Question 13Epidemiology

A factor that is associated with both the exposure and the outcome but is not on the causal pathway is called:

  1. A mediator
  2. A collider
  3. A confounder Correct answer.
  4. An effect modifier

Explanation

A confounder distorts the exposure-outcome relationship because it is related to both and is not intermediate. Adjustment or design strategies can control confounding.

Question 14Epidemiology

In an outbreak investigation, the first step after confirming an unusual increase in cases is typically to:

  1. Define and identify cases using a standard case definition Correct answer.
  2. Implement control measures immediately
  3. Close the implicated establishment
  4. Conduct a case-control study

Explanation

Establishing a clear case definition allows systematic identification and counting of cases before analytic steps. Control measures may begin early, but a case definition underpins the investigation.

Question 15Epidemiology

A vaccine has an efficacy of 80 percent. What does this mean?

  1. Eighty percent of vaccinated people will never be exposed to the pathogen
  2. Eighty percent of the population must be vaccinated to stop transmission
  3. Vaccinated people have an 80 percent lower risk of disease than unvaccinated people Correct answer.
  4. The vaccine causes protective antibodies in 80 percent of recipients

Explanation

Vaccine efficacy is the proportional reduction in disease risk among vaccinated compared with unvaccinated people, computed as 1 minus the risk ratio. It is distinct from coverage and from antibody response rates.

Question 16Epidemiology

Which measure best quantifies the excess disease burden in a population that could be eliminated if an exposure were removed?

  1. Population attributable fraction Correct answer.
  2. Odds ratio
  3. Relative risk
  4. Sensitivity

Explanation

The population attributable fraction combines the strength of association with exposure prevalence to estimate the proportion of disease in the population due to that exposure. Relative risk alone ignores how common the exposure is.

Question 17Epidemiology

Incidence density (incidence rate) differs from cumulative incidence primarily because it:

  1. Requires a fixed disease-free cohort
  2. Cannot be used in cohort studies
  3. Counts only fatal cases
  4. Accounts for person-time at risk Correct answer.

Explanation

Incidence density uses person-time in the denominator, allowing for varying follow-up and dynamic populations. Cumulative incidence uses the number at risk at the start of a fixed period.

Question 18Epidemiology

A study finds that the association between an exposure and disease differs substantially between men and women. This phenomenon is called:

  1. Effect modification Correct answer.
  2. Misclassification
  3. Selection bias
  4. Confounding

Explanation

Effect modification, or interaction, means the magnitude of the association varies across levels of a third variable such as sex. It is a real biological or social phenomenon to be reported, not a bias to remove.

Question 19Epidemiology

In a randomized controlled trial, randomization primarily serves to:

  1. Eliminate the need for informed consent
  2. Guarantee that the results apply to all populations
  3. Balance known and unknown confounders between groups Correct answer.
  4. Increase the precision of the outcome measurement

Explanation

Random assignment tends to balance both measured and unmeasured confounders across groups, supporting causal inference. It does not guarantee external validity or improve measurement precision.

Question 20Epidemiology

A screening program detects disease earlier and patients appear to live longer after diagnosis, but the time of death is unchanged. This apparent improvement is due to:

  1. Length bias
  2. Selection bias
  3. Recall bias
  4. Lead-time bias Correct answer.

Explanation

Lead-time bias occurs when earlier detection advances the diagnosis date without postponing death, inflating measured survival. Mortality rates, rather than survival time, are used to avoid this distortion.

Question 21Epidemiology

Which of the following best describes a notifiable disease surveillance system?

  1. A randomized survey of hospital admissions
  2. An insurance claims database used for billing
  3. A voluntary patient registry maintained by clinicians
  4. Mandatory reporting of specified diagnoses to public health authorities Correct answer.

Explanation

Notifiable disease surveillance relies on required reporting of designated conditions by clinicians and laboratories to health authorities. This enables monitoring and timely response.

Question 22Epidemiology

In a cohort of 1,000 people followed for 10 years, 50 develop a disease. Assuming complete follow-up, what is the cumulative incidence?

  1. 0.5 percent
  2. 10 percent
  3. 5 percent Correct answer.
  4. 50 percent

Explanation

Cumulative incidence is new cases divided by the population at risk at the start, so 50 divided by 1,000 equals 0.05, or 5 percent. It represents the average risk over the follow-up period.

Question 23Epidemiology

A case-control study of a rapidly fatal disease finds a weaker association than a cohort study of the same exposure. A plausible explanation is that:

  1. Case-control studies cannot measure exposure
  2. Survivors are overrepresented among cases, biasing exposure assessment Correct answer.
  3. The cohort study must have used a larger sample
  4. The disease is too rare for case-control designs

Explanation

When disease is rapidly fatal, prevalent cases may overrepresent people who survived long enough to be enrolled, distorting exposure comparisons. This is a form of selection bias related to survival.

Question 24Epidemiology

An investigator wants to estimate the effect of a treatment when confounding by indication is likely and randomization is infeasible. Which approach best supports causal inference?

  1. Reporting only the unadjusted relative risk
  2. Restricting the sample to patients who survived
  3. Crude comparison of treated and untreated patients
  4. Propensity score matching on measured confounders Correct answer.

Explanation

Propensity score methods balance measured confounders between treated and untreated groups, approximating the conditions of a randomized comparison. Residual confounding by unmeasured factors may still remain.

Question 25Epidemiology

In a population, a screening test with high specificity but low sensitivity is applied. Which outcome is most likely?

  1. Many healthy people will be falsely labeled positive
  2. Positive predictive value will be very low
  3. The test will have high yield for early detection
  4. Many true cases will be missed Correct answer.

Explanation

Low sensitivity means a substantial proportion of true cases test negative and are missed. High specificity means few false positives among healthy people.

Question 26Epidemiology

Which term describes the proportion of a population that has a disease at a specific point in time?

  1. Point prevalence Correct answer.
  2. Attack rate
  3. Incidence rate
  4. Case fatality rate

Explanation

Point prevalence counts all existing cases, both new and old, divided by the population at one moment. Incidence measures only new cases over a period, and case fatality measures deaths among cases.

Question 27Epidemiology

A community has 500 existing cases of a chronic disease among 50,000 residents. What is the point prevalence per 1,000?

  1. 1 per 1,000
  2. 0.1 per 1,000
  3. 10 per 1,000 Correct answer.
  4. 100 per 1,000

Explanation

Prevalence per 1,000 equals existing cases divided by population, multiplied by 1,000. Here 500 divided by 50,000 equals 0.01, and 0.01 times 1,000 equals 10.

Question 28Epidemiology

Which study design follows a defined group over time and measures the occurrence of new outcomes according to exposure status?

  1. Cross-sectional survey
  2. Cohort study Correct answer.
  3. Case-control study
  4. Ecologic study

Explanation

A cohort design starts with exposure status and observes participants forward in time to measure incident outcomes. This allows direct calculation of incidence and relative risk.

Question 29Epidemiology

Which measure of association is directly obtainable from a cohort study but not from a case-control study?

  1. Relative risk Correct answer.
  2. Odds ratio
  3. Attributable fraction in the exposed
  4. Prevalence ratio

Explanation

Cohort studies measure incidence in exposed and unexposed groups, so a true risk ratio can be computed. Case-control sampling fixes the number of cases and controls, so incidence cannot be directly estimated and only the odds ratio is available.

Question 30Epidemiology

In a cohort study, 30 of 300 exposed persons and 15 of 300 unexposed persons develop the outcome. What is the relative risk?

  1. 0.5
  2. 1.5
  3. 3.0
  4. 2.0 Correct answer.

Explanation

Risk in the exposed is 30/300 = 0.10 and risk in the unexposed is 15/300 = 0.05. Dividing 0.10 by 0.05 gives a relative risk of 2.0.

Question 31Epidemiology

A screening test has a sensitivity of 95 percent and a specificity of 90 percent. In a population with 10 percent prevalence, what is the approximate positive predictive value?

  1. 51 percent Correct answer.
  2. 90 percent
  3. 95 percent
  4. 10 percent

Explanation

In 1,000 people, 100 have disease, of whom 95 test positive. Of the 900 without disease, 90 test falsely positive, giving 185 total positives. Thus 95/185 is about 51 percent.

Question 32Epidemiology

Which characteristic distinguishes a pandemic from an epidemic?

  1. It is caused only by a novel pathogen
  2. It affects only a single household cluster
  3. It occurs at a predictable seasonal interval
  4. It spreads across multiple countries or continents Correct answer.

Explanation

A pandemic is an epidemic that has spread across several countries or continents, affecting a large number of people. An epidemic refers to an increase in cases above expected levels within a more limited area.

Question 33Epidemiology

Which measure expresses the number of new cases arising per unit of person-time at risk?

  1. Cumulative incidence
  2. Incidence density Correct answer.
  3. Point prevalence
  4. Proportionate mortality

Explanation

Incidence density divides new cases by the total person-time at risk, allowing for varying follow-up and competing risks. Cumulative incidence uses the size of the at-risk population at the start rather than person-time.

Question 34Epidemiology

A cohort of 2,000 persons is followed for a total of 8,000 person-years, during which 160 new cases occur. What is the incidence density per 1,000 person-years?

  1. 8 per 1,000 person-years
  2. 20 per 1,000 person-years Correct answer.
  3. 80 per 1,000 person-years
  4. 2 per 1,000 person-years

Explanation

Incidence density equals new cases divided by person-time, scaled to 1,000. Here 160 divided by 8,000 is 0.02, and 0.02 times 1,000 equals 20 per 1,000 person-years.

Question 35Epidemiology

Which type of bias arises when participants who volunteer for a study differ systematically from those who decline?

  1. Interviewer bias
  2. Self-selection bias Correct answer.
  3. Recall bias
  4. Misclassification bias

Explanation

Self-selection bias occurs when the decision to participate is linked to exposure or outcome, making volunteers unrepresentative of the target population. This affects the external validity and can distort internal comparisons as well.

Question 36Epidemiology

Investigators want to reduce confounding by age in a cohort analysis. Which approach restricts comparison to people of similar age?

  1. Randomization
  2. Matching or stratification Correct answer.
  3. Increasing sample size
  4. Blinding

Explanation

Matching at design or stratification at analysis compares exposed and unexposed persons within levels of the confounder. This removes the influence of age if the confounder is measured and controlled appropriately.

Question 37Epidemiology

A new rapid test is evaluated against a reference standard. Among 200 diseased persons, 160 test positive; among 800 non-diseased persons, 720 test negative. What is the specificity?

  1. 90 percent Correct answer.
  2. 80 percent
  3. 10 percent
  4. 20 percent

Explanation

Specificity is the proportion of non-diseased persons who test negative. Here 720 divided by 800 equals 0.90, or 90 percent.

Question 38Epidemiology

A case-control study of a rare exposure draws controls from patients attending the same hospital as the cases. Which bias is most concerning?

  1. Loss to follow-up
  2. Random measurement error
  3. Regression to the mean
  4. Selection bias from the control sampling Correct answer.

Explanation

Hospital-based controls may have conditions related to the exposure of interest, making them unrepresentative of the source population that produced the cases. This selection bias can distort the odds ratio in either direction.

Question 39Epidemiology

Which statement best explains why an ecologic correlation between average alcohol consumption and average liver disease rates does not prove an individual-level effect?

  1. The ecologic fallacy describes inferring individual associations from group-level data Correct answer.
  2. Group averages cannot be compared across regions
  3. Ecologic data are always measured with error
  4. Ecologic studies cannot calculate correlation coefficients

Explanation

The ecologic fallacy occurs when an association observed between group-level variables is assumed to hold at the individual level. Aggregate data cannot show whether the individuals with high exposure are the same ones with the outcome.

Question 40Epidemiology

In a randomized trial, the crude risk ratio is 1.8, but after stratifying by baseline severity the stratum-specific risk ratios are both about 1.0. Which interpretation is most appropriate?

  1. The result reflects random error only
  2. Baseline severity confounds the crude association Correct answer.
  3. The exposure has a strong causal effect
  4. The exposure is protective in both strata

Explanation

When stratum-specific estimates differ from the crude estimate and are homogeneous with each other, the crude association is confounded by the stratifying variable. Baseline severity is associated with both exposure and outcome, so adjusting for it removes the apparent effect.

Question 41Biostatistics

Which measure of disease frequency expresses the number of new cases occurring in a population at risk during a specified period?

  1. Attack rate ratio
  2. Incidence rate Correct answer.
  3. Point prevalence
  4. Case fatality rate

Explanation

Incidence quantifies new cases arising in a population at risk over a defined time period. Prevalence counts all existing cases at a point or interval, regardless of when they arose.

Question 42Biostatistics

A screening test has high sensitivity but low specificity. What is the most likely consequence when applied in a low-prevalence population?

  1. A reduced positive predictive value only for diseased persons
  2. A large number of false negatives
  3. A large number of false positives Correct answer.
  4. A high negative predictive value with few false positives

Explanation

Low specificity means many non-diseased people test positive, and in a low-prevalence setting these false positives dominate the positives. Sensitivity affects detection of true cases, not the false-positive burden.

Question 43Biostatistics

In a case-control study, which measure of association is calculated directly from the data?

  1. Incidence rate ratio
  2. Attributable risk percent
  3. Risk ratio
  4. Odds ratio Correct answer.

Explanation

Case-control sampling fixes the number of cases and controls, so incidence cannot be estimated and the odds ratio is the appropriate measure. The odds ratio approximates the risk ratio only when the outcome is rare.

Question 44Biostatistics

A study reports a 95% confidence interval for a risk ratio of 1.8 (95% CI 1.2 to 2.7). Which interpretation is correct?

  1. There is a 95% probability that the true value lies in this specific interval
  2. The result is not statistically significant because the interval is wide
  3. The true risk ratio is exactly 1.8 in the population
  4. The association is statistically significant at the 0.05 level Correct answer.

Explanation

Because the interval excludes the null value of 1.0, the association is statistically significant at alpha 0.05. The interval describes the range of plausible values under repeated sampling, not a probability statement about one interval.

Question 45Biostatistics

A researcher wants to compare mean blood pressure across three independent groups. Which test is most appropriate?

  1. Chi-square test
  2. Paired t-test
  3. McNemar test
  4. One-way analysis of variance Correct answer.

Explanation

ANOVA compares means across two or more independent groups while controlling the overall type I error rate. Multiple pairwise t-tests would inflate the chance of a false positive.

Question 46Biostatistics

In a randomized controlled trial, what is the primary purpose of randomization?

  1. To increase the sample size
  2. To balance known and unknown confounders between groups Correct answer.
  3. To guarantee that the trial is blinded
  4. To eliminate measurement error

Explanation

Random assignment tends to distribute both measured and unmeasured confounders evenly across groups, supporting causal inference. It does not enlarge samples, reduce measurement error, or ensure blinding.

Question 47Biostatistics

A test with a sensitivity of 0.90 and specificity of 0.95 is applied to 1,000 people, of whom 100 have the disease. How many false positives are expected?

  1. 55
  2. 90
  3. 10
  4. 45 Correct answer.

Explanation

Of 900 non-diseased people, specificity 0.95 means 5% test positive, giving 45 false positives. False negatives would come from the diseased group and sensitivity.

Question 48Biostatistics

Which measure best describes the strength of agreement between two observers classifying the same subjects into categories beyond chance?

  1. Cohen's kappa Correct answer.
  2. Cronbach's alpha
  3. Pearson correlation
  4. Coefficient of variation

Explanation

Cohen's kappa adjusts observed agreement for the agreement expected by chance alone. It is the standard statistic for inter-rater agreement on categorical ratings.

Question 49Biostatistics

A p-value of 0.03 from a hypothesis test indicates which of the following?

  1. The probability that the result will replicate
  2. The probability that the null hypothesis is true
  3. The magnitude of the treatment effect
  4. The probability of obtaining data at least as extreme as observed, assuming the null hypothesis is true Correct answer.

Explanation

A p-value is the tail probability of the observed or more extreme result under the null hypothesis. It is not the probability that the null is true, nor a measure of effect size or replicability.

Question 50Biostatistics

In a cohort study of 500 exposed and 500 unexposed workers, 50 exposed and 25 unexposed develop a condition. What is the risk ratio?

  1. 0.5
  2. 1.5
  3. 2.0 Correct answer.
  4. 0.25

Explanation

Risk in exposed is 50/500 = 0.10 and in unexposed is 25/500 = 0.05, so the risk ratio is 0.10/0.05 = 2.0. The ratio compares incidence between groups.