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CPH (Other exams) practice questions

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Question 1Public Health Sciences

Which measure of disease frequency is defined as the number of new cases occurring in a population at risk during a specified time period divided by the person-time at risk?

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

Explanation

An incidence rate expresses new cases per unit of person-time at risk, capturing the speed at which new cases arise. Prevalence counts existing cases at a point or period, while case fatality is a proportion of deaths among cases.

Question 2Public Health Sciences

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

  1. Few false positives and high positive predictive value
  2. Many false negatives and low negative predictive value
  3. Many false positives and low positive predictive value Correct answer.
  4. No effect on predictive values

Explanation

High sensitivity means few false negatives, but low specificity produces many false positives. When prevalence is low, those false positives dominate the test-positive group, driving positive predictive value down.

Question 3Public Health Sciences

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

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

Explanation

Case-control sampling fixes the number of cases and controls, so incidence cannot be measured directly; the odds ratio approximates the risk ratio when disease is rare. Risk ratios and incidence rate ratios require knowing the denominators of exposed and unexposed groups.

Question 4Public Health Sciences

Which type of bias arises when participants who are lost to follow-up differ systematically from those who remain in a cohort study?

  1. Interviewer bias
  2. Lead-time bias
  3. Attrition bias Correct answer.
  4. Recall bias

Explanation

Attrition bias occurs when differential loss of participants is related to both exposure and outcome, distorting the observed association. Recall and interviewer bias concern how information is gathered, and lead-time bias concerns earlier detection through screening.

Question 5Public Health Sciences

A randomized controlled trial reports a relative risk of 0.75 (95% CI 0.60 to 0.94) for a new drug versus placebo. Which interpretation is most appropriate?

  1. The drug significantly reduces risk because the confidence interval excludes 1.0 Correct answer.
  2. The drug increases risk because the upper bound is below 1.0
  3. The result is inconclusive because the interval is wide
  4. The drug has no effect because the point estimate is below 1.0

Explanation

A confidence interval for a relative risk that excludes the null value of 1.0 indicates a statistically significant association at the conventional level. Here the entire interval lies below 1.0, consistent with a protective effect.

Question 6Public Health Sciences

Which of the following best describes the purpose of a randomized controlled trial?

  1. To measure disease prevalence in the community
  2. To guarantee that results generalize to all populations
  3. To balance known and unknown confounders between comparison groups Correct answer.
  4. To eliminate the need for informed consent

Explanation

Random allocation tends to distribute both measured and unmeasured confounders evenly across groups, supporting causal inference. It does not remove the need for consent, ensure external validity, or measure prevalence.

Question 7Public Health Sciences

Which factor most directly increases the statistical power of a study?

  1. Increasing the sample size Correct answer.
  2. Decreasing the effect size
  3. Reducing the significance level from 0.05 to 0.01
  4. Increasing measurement error

Explanation

Power rises as sample size increases because larger samples reduce random error and standard error. Stricter significance thresholds, greater measurement error, and smaller true effects all reduce power.

Question 8Public Health Sciences

In a 2x2 table, the number of true positives is 80, false positives 20, false negatives 20, and true negatives 880. What is the sensitivity?

  1. 0.90
  2. 0.80 Correct answer.
  3. 0.50
  4. 0.20

Explanation

Sensitivity is true positives divided by all people with disease (true positives plus false negatives), or 80/100 = 0.80. Specificity would use true negatives over all without disease.

Question 9Public Health Sciences

Which study design is best suited to establish temporality between an exposure and an outcome?

  1. Cross-sectional survey
  2. Prospective cohort study Correct answer.
  3. Case series
  4. Ecological study

Explanation

A prospective cohort measures exposure before outcome occurrence, establishing that exposure precedes disease. Cross-sectional, ecological, and case-series designs generally cannot confirm temporal order at the individual level.

Question 10Public Health Sciences

A community program reports that 40% of adults are current smokers. This figure is best described as which type of measure?

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

Explanation

A proportion of existing smokers at a point in time is a prevalence proportion. Incidence rate measures new cases per person-time, and mortality and case fatality concern deaths.

Question 11Public Health Sciences

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

  1. Rehabilitation after a stroke
  2. Chemotherapy for diagnosed leukemia
  3. Immunization of children against measles Correct answer.
  4. Screening for colorectal cancer

Explanation

Primary prevention acts before disease occurs, such as vaccination to prevent infection. Screening detects disease early (secondary), while rehabilitation and treatment address existing disease (tertiary).

Question 12Public Health Sciences

Herd immunity primarily protects susceptible individuals by which mechanism?

  1. Reducing the probability that a susceptible person contacts an infectious case Correct answer.
  2. Eliminating the need for any vaccination
  3. Preventing all infections in vaccinated persons
  4. Increasing the virulence of the pathogen over time

Explanation

When a sufficient proportion of a population is immune, chains of transmission are interrupted and susceptible people are less likely to encounter an infectious case. Herd immunity does not eliminate the need for vaccination or guarantee protection for every vaccinated person.

Question 13Public Health Sciences

Which measure of association is calculated as (risk in exposed minus risk in unexposed) divided by risk in exposed?

  1. Attributable fraction among the exposed Correct answer.
  2. Odds ratio
  3. Population attributable fraction
  4. Relative risk

Explanation

The attributable fraction among the exposed expresses the proportion of disease in the exposed group that could be avoided if exposure were removed. The population attributable fraction uses total population risk instead of exposed-group risk.

Question 14Public Health Sciences

In testing a hypothesis, a type I error occurs when which of the following happens?

  1. A true null hypothesis is rejected Correct answer.
  2. The sample size is too small
  3. The confidence interval excludes zero
  4. A false null hypothesis is retained

Explanation

A type I error is rejecting a null hypothesis that is actually true, corresponding to a false positive result. A type II error is failing to reject a false null hypothesis.

Question 15Public Health Sciences

Which of the following best describes an ecological fallacy?

  1. Using person-time denominators in incidence rates
  2. Inferring individual-level associations from group-level data Correct answer.
  3. Applying random assignment in a community trial
  4. Assuming exposure precedes outcome in a cohort

Explanation

The ecological fallacy is the error of concluding that group-level associations hold at the individual level. Ecological data can suggest hypotheses but cannot confirm individual-level relationships.

Question 16Public Health Sciences

A new diagnostic test is evaluated against a gold standard. Which property is independent of disease prevalence in the tested population?

  1. Positive predictive value
  2. Specificity Correct answer.
  3. Negative predictive value
  4. Post-test probability of disease

Explanation

Sensitivity and specificity are intrinsic test characteristics that do not change with prevalence. Predictive values and post-test probabilities depend on the pre-test probability of disease in the population.

Question 17Public Health Sciences

Which method is used to control for confounding at the design stage of a study?

  1. Multivariable regression
  2. Standardization of rates
  3. Stratified analysis
  4. Restriction of study participants to a single category of the confounder Correct answer.

Explanation

Restriction, along with matching, controls confounding during design by limiting variation in the confounder. Stratification, regression, and standardization are analytic approaches applied after data collection.

Question 18Public Health Sciences

A cohort study finds an incidence of 10 per 1000 person-years among exposed and 5 per 1000 person-years among unexposed. What is the incidence rate ratio?

  1. 5.0
  2. 2.0 Correct answer.
  3. 10.0
  4. 0.5

Explanation

The incidence rate ratio is the exposed rate divided by the unexposed rate, or 10/5 = 2.0, indicating twice the rate among exposed persons.

Question 19Public Health Sciences

Which concept explains why survival appears longer when a disease is detected by screening rather than by symptoms, even if treatment is equally effective?

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

Explanation

Lead-time bias occurs because screening advances the time of diagnosis, inflating measured survival without extending life. Length bias refers to preferential detection of slower-progressing disease.

Question 20Public Health Sciences

In a randomized trial, the intention-to-treat analysis compares participants according to which criterion?

  1. Their final treatment status at the end of the study
  2. The group to which they were originally assigned Correct answer.
  3. Whether they adhered to the assigned treatment
  4. Their baseline risk profile

Explanation

Intention-to-treat analysis preserves randomization by analyzing participants in their assigned groups regardless of adherence. Per-protocol analysis, by contrast, restricts to those who followed the protocol.

Question 21Public Health Sciences

Which of the following is a key characteristic of a cross-sectional study?

  1. Exposure is randomly assigned
  2. Exposure and outcome are measured at the same time Correct answer.
  3. Participants are selected based on outcome status
  4. Participants are followed prospectively for many years

Explanation

Cross-sectional studies measure exposure and outcome simultaneously, yielding prevalence rather than incidence. They cannot establish temporal order or test causal direction.

Question 22Public Health Sciences

A screening program detects a disease earlier but does not change the age at death. Which outcome has improved?

  1. Both survival time and mortality rate
  2. Survival time from diagnosis but not mortality rate Correct answer.
  3. Neither survival time nor mortality rate
  4. Mortality rate but not survival time from diagnosis

Explanation

Earlier diagnosis lengthens the interval from diagnosis to death (lead time) without postponing death, so survival from diagnosis improves while the population mortality rate is unchanged.

Question 23Public Health Sciences

Which of the following best defines the median of a dataset?

  1. The difference between the largest and smallest values
  2. The value that divides the ordered observations into two equal halves Correct answer.
  3. The arithmetic average of all values
  4. The most frequently occurring value

Explanation

The median is the middle value of an ordered dataset, dividing it into two halves. The mode is the most frequent value, the mean is the arithmetic average, and the range is the spread.

Question 24Public Health Sciences

Which of the following is the most appropriate measure of central tendency for a strongly right-skewed distribution?

  1. Range
  2. Mode
  3. Median Correct answer.
  4. Mean

Explanation

The median is resistant to extreme values and better represents the center of a skewed distribution. The mean is pulled toward the tail, and the range is a measure of spread rather than center.

Question 25Public Health Sciences

Which of the following is a core function of public health surveillance?

  1. Systematic collection and analysis of health data to guide action Correct answer.
  2. Individual clinical diagnosis and treatment
  3. Randomized testing of new pharmaceuticals
  4. Setting individual patient treatment goals

Explanation

Surveillance involves ongoing systematic collection, analysis, and dissemination of health data to inform public health action. Clinical diagnosis, drug trials, and individual treatment planning are not surveillance functions.

Question 26Public Health Sciences

Which of the following best describes the epidemiologic concept of incidence?

  1. The proportion of deaths among all persons diagnosed with a disease
  2. The number of new cases of a disease that develop in a population at risk during a specified period Correct answer.
  3. The total number of existing cases of a disease at a single point in time
  4. The average duration of a disease from onset to recovery

Explanation

Incidence measures the occurrence of new cases arising in a population at risk over a defined time period, capturing the transition from disease-free to diseased. Prevalence, by contrast, counts all existing cases at a given moment.

Question 27Public Health Sciences

A public health department wants to compare death rates between two populations while removing the effect of differing age structures. Which method is appropriate?

  1. Computing the case-fatality ratio for each population
  2. Direct standardization using a common reference age distribution Correct answer.
  3. Reporting the total number of deaths in each population
  4. Calculating crude death rates for each population

Explanation

Age standardization applies a common reference population's age weights to each group's age-specific rates, removing confounding by age structure. Crude rates and raw counts remain distorted by differing age distributions.

Question 28Public Health Sciences

Which of the following is the primary purpose of a disease registry in public health practice?

  1. To estimate the prevalence of all health conditions in the population simultaneously
  2. To randomly assign participants to treatment and control groups
  3. To replace vital statistics records with clinical data
  4. To systematically collect and track cases of a defined condition over time for monitoring and planning Correct answer.

Explanation

A registry is an organized system that continuously collects defined data on persons with a specific condition, supporting surveillance, quality improvement, and service planning. It does not assign treatments or attempt to capture all conditions at once.

Question 29Public Health Sciences

A researcher wants to reduce selection bias when recruiting participants for a case-control study. Which approach best addresses this concern?

  1. Allowing participants to self-select into the study
  2. Recruiting cases from a referral center that serves a distinct region
  3. Selecting controls only from patients hospitalized for unrelated conditions
  4. Selecting controls from the same source population that gave rise to the cases Correct answer.

Explanation

Choosing controls from the same underlying population as cases ensures that control selection reflects the exposure distribution of the source population, minimizing selection bias. Self-selection and divergent referral sources introduce systematic differences unrelated to disease.

Question 30Public Health Sciences

Which of the following is a defining feature of a case series report?

  1. It randomly allocates participants to interventions
  2. It describes characteristics of a group of patients with a condition without a comparison group Correct answer.
  3. It calculates incidence rates using person-time denominators
  4. It estimates the relative risk of an exposure using prospective follow-up

Explanation

A case series is a descriptive account of a collection of patients with a condition, lacking a comparison group and therefore unable to estimate associations. Incidence rates, randomization, and relative risks require analytic designs.

Question 31Public Health Sciences

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. 2.0 Correct answer.
  2. 0.10
  3. 0.5
  4. 25

Explanation

Risk in exposed is 50/500 = 0.10 and risk in unexposed is 25/500 = 0.05. The risk ratio is 0.10/0.05 = 2.0, indicating twice the risk among the exposed.

Question 32Public Health Sciences

Which of the following best characterizes the role of a confidence interval in interpreting an epidemiologic estimate?

  1. It proves that the point estimate is the true population value
  2. It indicates the range of values compatible with the data at a chosen confidence level Correct answer.
  3. It eliminates the effect of random error in the study
  4. It measures the probability that the null hypothesis is true

Explanation

A confidence interval expresses the precision of an estimate by giving a range of plausible values around the point estimate at a stated confidence level. It does not prove the true value, give the probability of the null hypothesis, or remove random error.

Question 33Public Health Sciences

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

  1. Legislation banning smoking in public places
  2. Immunization of children against measles
  3. Rehabilitation following a stroke
  4. Mammography screening to detect breast cancer at an early stage Correct answer.

Explanation

Secondary prevention aims to detect disease early in asymptomatic individuals so that treatment can alter its course; screening programs such as mammography fit this definition. Immunization and smoking bans prevent disease onset, while rehabilitation addresses existing disease.

Question 34Public Health Sciences

Which of the following best describes the concept of attributable risk in a population?

  1. The proportion of disease in the population that could be eliminated if the exposure were removed Correct answer.
  2. The ratio of disease risk among exposed to risk among unexposed
  3. The proportion of exposed persons who develop disease because of the exposure
  4. The number of exposures required to prevent one additional case

Explanation

Population attributable risk estimates the fraction of disease burden in the population linked to an exposure and thus potentially preventable by removing it. The risk ratio and exposed-specific attributable fraction address different questions.

Question 35Public Health Sciences

Which of the following best defines the positive predictive value of a screening test?

  1. The proportion of persons with a positive result who actually have the disease Correct answer.
  2. The proportion of persons without the disease who test negative
  3. The proportion of all tested persons who test positive
  4. The proportion of persons with the disease who test positive

Explanation

Positive predictive value is the probability of disease given a positive test result. It depends on both test sensitivity and specificity and on the prevalence of disease in the tested population.

Question 36Public Health Sciences

A screening test has sensitivity of 90% and specificity of 95%. When the same test is applied in a population with much lower disease prevalence, which measure will decrease?

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

Explanation

Positive predictive value falls as prevalence decreases because fewer true cases exist among those testing positive, so a larger share of positives are false positives. Sensitivity and specificity are intrinsic test properties that do not change with prevalence.

Question 37Public Health Sciences

Which of the following best describes the purpose of blinding in a clinical trial?

  1. To eliminate confounding by measured baseline variables
  2. To reduce bias arising from participants' and investigators' expectations about treatment assignment Correct answer.
  3. To ensure that all participants receive the active treatment
  4. To increase the sample size needed to detect an effect

Explanation

Blinding conceals treatment assignment so that expectations do not influence reporting, behavior, or outcome assessment, thereby limiting information and observer bias. It does not affect sample size requirements, guarantee active treatment, or address measured confounding.

Question 38Public Health Sciences

A trial randomizes participants but 15% of the intervention group never take the assigned treatment. Which analysis best preserves the benefits of randomization?

  1. Per-protocol analysis
  2. As-treated analysis
  3. Complete-case analysis excluding non-adherent participants
  4. Intention-to-treat analysis Correct answer.

Explanation

Intention-to-treat analysis compares groups as originally randomized, preserving the comparability created by randomization and avoiding bias from post-randomization selection. Excluding or regrouping non-adherent participants can reintroduce confounding.

Question 39Public Health Sciences

Which of the following best describes confounding in an epidemiologic study?

  1. Loss of participants during follow-up
  2. A third factor associated with both the exposure and the outcome that distorts their observed relationship Correct answer.
  3. Random variation in the measurement of the exposure
  4. Differential recall of exposure between cases and controls

Explanation

A confounder is a variable related to both exposure and outcome and not on the causal pathway, which can create or mask an apparent association. Measurement error, recall differences, and attrition are separate sources of bias.

Question 40Public Health Sciences

Which of the following is the most appropriate measure of association for a matched case-control study?

  1. Incidence rate ratio
  2. Mantel-Haenszel odds ratio for matched pairs Correct answer.
  3. Crude risk ratio
  4. Cumulative incidence difference

Explanation

Matched case-control designs require analysis that accounts for the matching, such as the Mantel-Haenszel estimator for matched pairs or conditional logistic regression. Risk ratios and incidence rates cannot be directly estimated from case-control data because the underlying denominators are unknown.

Question 41Public Health Sciences

Which of the following best defines the reproductive number (R0) of an infectious disease?

  1. The time between exposure and onset of infectiousness
  2. The proportion of a population that is immune to an infection
  3. The case-fatality proportion of an infectious disease
  4. The average number of secondary cases generated by one infectious case in a fully susceptible population Correct answer.

Explanation

R0 describes the expected number of secondary infections produced by a single infectious individual introduced into an entirely susceptible population. It is a property of the pathogen and contact patterns, not of immunity levels or case fatality.

Question 42Public Health Sciences

An outbreak investigation finds that illness is strongly associated with a specific food item served at a banquet. Which step is most appropriate next?

  1. Conclude the investigation and close the case
  2. Assume the association is coincidental until a randomized trial is conducted
  3. Wait for laboratory confirmation before removing the food item
  4. Implement control measures while continuing to investigate the source Correct answer.

Explanation

In outbreak response, control measures should be implemented as soon as strong epidemiologic evidence identifies a likely source, without waiting for full laboratory confirmation. Delaying action allows continued exposure and additional cases.

Question 43Public Health Sciences

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

  1. To require health care providers and laboratories to report specified conditions to public health authorities Correct answer.
  2. To provide clinical treatment guidelines for infectious diseases
  3. To randomly sample households for health interviews
  4. To reimburse hospitals for the cost of treating reportable conditions

Explanation

Notifiable disease systems mandate reporting of designated conditions so that public health agencies can detect trends, investigate outbreaks, and direct control measures. They are not treatment guidelines, payment mechanisms, or sampling surveys.

Question 44Public Health Sciences

Which of the following is the best example of a modifiable risk factor targeted by a population-level cardiovascular prevention strategy?

  1. Genetic predisposition to hyperlipidemia
  2. Family history of coronary disease
  3. Tobacco use Correct answer.
  4. Age

Explanation

Tobacco use can be changed through policy, education, and cessation support, making it a modifiable risk factor suitable for population-level intervention. Age, family history, and inherited predisposition cannot be altered.

Question 45Public Health Sciences

Which of the following best describes the difference between health equity and health equality?

  1. Equity focuses only on genetic factors, whereas equality focuses on environmental factors
  2. Equity guarantees identical health outcomes, whereas equality allows variation
  3. Equity allocates resources according to need, whereas equality allocates the same resources to everyone Correct answer.
  4. Equity applies only to infectious diseases, whereas equality applies to chronic diseases

Explanation

Health equity recognizes that different groups may need different levels of support to reach their full health potential, while equality means distributing identical resources regardless of need. Equity targets avoidable, unjust differences in health.

Question 46Public Health Sciences

Which of the following is the most appropriate summary measure of disease burden that accounts for both premature death and disability?

  1. Life expectancy at birth
  2. Disability-adjusted life years Correct answer.
  3. Case-fatality ratio
  4. Crude mortality rate

Explanation

Disability-adjusted life years combine years of life lost to premature mortality with years lived with disability, providing a single metric of overall disease burden. Crude mortality, case fatality, and life expectancy each capture only part of the picture.

Question 47Public Health Sciences

A community health assessment finds that a neighborhood has limited access to fresh produce and high rates of diet-related disease. Which term best describes this situation?

  1. A randomized crossover design
  2. A food desert Correct answer.
  3. A case series
  4. A point-source outbreak

Explanation

A food desert is an area with limited access to affordable, nutritious food, which contributes to diet-related disease. The other terms describe outbreak patterns or study designs, not neighborhood food environments.

Question 48Public Health Sciences

Which of the following best describes the role of a sensitivity analysis in epidemiologic modeling?

  1. It calculates the statistical power of a study before data collection
  2. It determines the sample size required for a trial
  3. It removes all bias from observational data
  4. It tests how conclusions change when assumptions or input values are varied Correct answer.

Explanation

Sensitivity analysis examines the robustness of results by systematically varying assumptions, parameter values, or analytic choices. It does not replace power calculations, eliminate bias, or set sample size.

Question 49Public Health Sciences

Which of the following is the most appropriate measure of association to report from a cross-sectional study?

  1. Risk difference over time
  2. Prevalence ratio Correct answer.
  3. Incidence rate ratio
  4. Number needed to treat

Explanation

Cross-sectional studies measure exposure and outcome simultaneously and therefore estimate prevalence, making the prevalence ratio the appropriate measure. Incidence-based measures and number needed to treat require longitudinal follow-up or trial data.

Question 50Public Health Sciences

Which of the following best describes the concept of lead-time bias in screening evaluation?

  1. Participants in screening programs are healthier than non-participants
  2. Apparent survival is lengthened because diagnosis occurs earlier, even if the time of death is unchanged Correct answer.
  3. Screening detects slowly progressive disease that would never have caused symptoms
  4. The screening test misses a substantial proportion of true cases

Explanation

Lead-time bias arises because screening advances the date of diagnosis, inflating measured survival time from diagnosis even when the actual time of death is unaffected. It must be distinguished from length bias and selection bias.