Free DACVPM (Epidemiology) Practice Questions

Question 1

Groups visiting a dairy farm on May 3 and May 10 develop enteric illness, producing two peaks seven days apart on a curve plotted by symptom onset. The identified organism has an incubation period of 1-3 days in this investigation. Cases in each group began 1-3 days after that group's visit, and illness in both groups was concentrated among visitors who drank the farm's milk. Investigators find no links between the groups through households, transport, or social contact. Which explanation of the two peaks is best supported?

Accepted Answer

Repeated exposure to a common source during the two farm visits.

Answer

Ill visitors in the first group infected visitors in the second group.

Answer

A single exposure on May 3 produced two widely separated incubation groups.

Answer

Delayed notification divided one continuous cluster of illness onsets into two peaks.

Question 2

A clinically normal breeding pig has a positive infection screen. Among clinically normal breeding pigs in this source population, infection prevalence is 1%. The assay has 90% sensitivity and 98% specificity for that population, established against a valid reference standard. The owner interprets the high specificity as meaning that this positive result makes infection almost certain. Which response correctly uses the probability of infection after a positive result?

Accepted Answer

About 31%; a positive result is more likely to be false than true in this population.

Answer

About 69%; most positive results represent infection despite the low background prevalence.

Answer

About 90%; sensitivity gives the probability that a pig with a positive result is infected.

Answer

About 98%; specificity gives the probability that a pig with a positive result is infected.

Question 3

In a dairy cohort, each first clinical mastitis case is matched to four controls sampled from cows still free of clinical mastitis on that farm on the case's onset date. Exposure histories are assessed before that date, and the matched risk sets are retained for analysis. A March control develops mastitis in June. A reviewer requests deletion of her March record to prevent overlap between cases and controls. How should her records be handled?

Accepted Answer

Retain the March control record and include the June event as an incident case.

Answer

Replace her March control record with a cow disease-free at study end.

Answer

Keep the March record but omit the June event so that the cow contributes to only one outcome group.

Answer

Classify her March record as a case, using the eventual diagnosis.

Question 4

West Nile virus infection is confirmed in horses at several unconnected stables in one district. Mosquito pools from the district also test positive, and surveillance has recorded infections in wild birds. The stables have not exchanged horses, equipment, or personnel. When setting the district's central control priority, the veterinary and public-health team should:

Accepted Answer

Reduce mosquito exposure and use bird and vector findings to direct local risk assessment.

Answer

Recommend suspending movements of all local horses until every stable has completed negative herd testing.

Answer

Trace horse-to-horse contacts to interrupt the local transmission cycle.

Answer

Screen horses for viremia to locate the main reservoir supplying infection to the district's mosquitoes.

Question 5

A livestock survey includes 200-animal and 800-animal herds. Their respective probabilities of herd selection are 0.10 and 0.40. Within each selected herd, 20 animals are selected by simple random sampling, and every selected animal is tested. An analyst proposes weighting small-herd animals four times as heavily as large-herd animals. Overall animal inclusion probability is the product of the herd-selection and within-herd selection probabilities. For animal-level prevalence, what should the analysis use?

Accepted Answer

Equal animal weights, with herd structure retained in the variance analysis.

Answer

The proposed fourfold weights, with animals treated as independent observations.

Answer

Equal animal weights, treating animals as independent in variance estimation.

Answer

Fourfold weights for large-herd animals, with herd structure retained for variance estimation.

Question 6

Thirty-six pig farms are randomized to two sanitation protocols. Forty pigs per farm are assessed once for diarrhea, and outcomes are positively correlated within farms. An industry report presents a narrow confidence interval from ordinary logistic regression treating all pigs as independent. Before accepting that claimed precision, what reanalysis should be requested?

Accepted Answer

Account for farm-level clustering in the treatment comparison and its uncertainty.

Answer

Apply Fisher's exact test to the pooled treatment-by-diarrhea table.

Answer

Retain independent-animal analysis because the same number of pigs was assessed on each farm.

Answer

Adjust for each pig's initial weight while retaining independent binomial errors in the fitted model.

Question 7

A herd-randomized udder-hygiene trial reports 12-week mastitis risks separately for two baseline-risk strata. In higher-risk herds, risk is 20% with usual practice and 10% with the program: risk ratio 0.50, absolute reduction 10 percentage points. In lower-risk herds, the corresponding figures are 4% and 2%: risk ratio 0.50, absolute reduction 2 percentage points. The analysis accounts for herd assignment. A coauthor writes, 'The program works equally well in both strata.' Which revision accurately describes these point estimates?

Accepted Answer

Absolute benefit differs between strata; relative benefit is the same.

Answer

Matching risk ratios demonstrate equal benefit on both scales.

Answer

Higher-risk herds receive greater relative protection, while the absolute protection is equal.

Answer

The estimates should be pooled because randomization removes differences in treatment response.

Question 8

Reviewing a racehorse study, an epidemiologist finds that age was adjusted for as younger than five years versus five years or older. Within both categories, horses receiving the exercise regimen are older than comparison horses. The investigators' causal model identifies age as influencing regimen selection and independently increasing injury risk. Exact ages were recorded. Which reanalysis most directly addresses the remaining age-related distortion?

Accepted Answer

Model age more finely using the recorded exact ages.

Answer

Add horses while retaining the same two-category age adjustment.

Answer

Test an age-category-by-regimen interaction before reconsidering age adjustment.

Answer

Adjust for fitness improvement produced by the regimen during follow-up.

Question 9

Two districts obtain zero positives in surveys planned for 95% sensitivity at a 2% design prevalence. Their documented histories give them different prior probabilities of infection, neither zero nor one. The probability of a negative survey given infection is the same across districts; so is the probability of a negative survey given freedom. Negative evidence favors freedom but cannot exclude infection. In the Bayesian updates, which relationship must hold?

Accepted Answer

Their prior odds of infection are multiplied by the same factor.

Answer

Both districts' infection probabilities decrease by the same proportion.

Answer

Their probabilities of freedom increase by the same number of percentage points.

Answer

Their probabilities of freedom both equal 95%, as determined by surveillance sensitivity.

Question 10

A dairy cooperative is evaluating an automated pen-washing program to reduce calf diarrhea. Baseline stocking density influences both adoption of the program and diarrhea risk. The program lowers environmental contamination after installation, and that reduction contributes to its effect on diarrhea. The study's causal model identifies stocking density as the only common cause of adoption and diarrhea; both program choices occur at every density level represented. To estimate the program's total effect, which variables belong in the adjustment set?

Accepted Answer

Baseline stocking density only.

Answer

Baseline stocking density and post-installation contamination.

Answer

Post-installation contamination only, to isolate the program's effect.

Answer

Neither variable, because the comparison concerns the entire program.

Free DACVPM (Epidemiology) Practice Questions

Domain 1: Causality

Question 1

Domain 3: Measures of Association

Question 2

Domain 4: Data Acquisition, Organization, and Presentation

Question 3

Domain 5: Epidemiologic Study Designs

Question 4

Domain 7: Error and Confounding

Question 5

Domain 8: Conceptual Understanding, Interpretation/Application of Diagnostic Tests

Question 6

Domain 9: Epidemiologic Surveillance

Question 7

Domain 10: Outbreak Investigation

Question 8

Domain 11: Infectious Disease Epidemiology/Modeling

Question 9

Domain 12: Biostatistics

Question 10

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