Lesson 1: Introduction to Epidemiology

Answers to Self-Assessment Quiz

1. A, B, C. In the definition of epidemiology, “distribution” refers to descriptive epidemiology, while “determinants” refers to analytic epidemiology. So “distribution” covers time (when), place (where), and person (who), whereas “determinants” covers causes, risk factors, modes of transmission (why and how).
2. A, B, D, E. In the definition of epidemiology, “determinants” generally includes the causes (including agents), risk factors (including exposure to sources), and modes of transmission, but does not include the resulting public health action.
3. A, C, D. Epidemiology includes assessment of the distribution (including describing demographic characteristics of an affected population), determinants (including a study of possible risk factors), and the application to control health problems (such as closing a restaurant). It does not generally include the actual treatment of individuals, which is the responsibility of health-care providers.
4. A, B, D, E. John Snow’s investigation of cholera is considered a model for epidemiologic field investigations because it included a biologically plausible (but not popular at the time) hypothesis that cholera was water-borne, a spot map, a comparison of a health outcome (death) among exposed and unexposed groups, and a recommendation for public health action. Snow’s elegant work predated multivariate analysis by 100 years.
5. B, C, D. Public health surveillance includes collection (B), analysis (C), and dissemination (D) of public health information to help guide public health decision making and action, but it does not include individual clinical diagnosis, nor does it include the actual public health actions that are developed based on the information.
6. A. The hallmark feature of an analytic epidemiologic study is use of an appropriate comparison group.
7. A. A case definition for a field investigation should include clinical criteria, plus specification of time, place, and person. The case definition should be independent of the exposure you wish to evaluate. Depending on the availability of laboratory confirmation, certainty of diagnosis, and other factors, a case definition may or may not be developed for suspect cases. The nationally agreed standard case definition for disease reporting is usually quite specific, and usually does not include suspect or possible cases.
8. A, D. A specific or tight case definition is one that is likely to include only (or mostly) true cases, but at the expense of excluding milder or atypical cases.
9. C. Rates assess risk. Numbers are generally preferred for identifying individual cases and for resource planning.
10. B. An epidemic curve, with date or time of onset on its x-axis and number of cases on the y-axis, is the classic graph for displaying the time course of an epidemic.
11. A, B, C. “Place” includes location of actual or suspected exposure as well as location of residence, work, school, and the like.
12. A, C. “Person” refers to demographic characteristics. It generally does not include clinical features characteristics or exposures.
13. D. Epidemiologists tailor descriptive epidemiology to best describe the data they have. Because different diseases have different age distributions, epidemiologists use different age breakdowns appropriate for the disease of interest.
14. A, E. A study in which subjects are randomized into two intervention groups and monitored to identify health outcomes is a clinical trial, which is type of experimental study. It is not a cohort study, because that term is limited to observational studies.
15. B, C. A study that assesses (but does not dictate) exposure and follows to document subsequent occurrence of disease is an observational cohort study.
16. B, D. A study in which subjects are enrolled on the basis of having or not having a health outcome is an observational case-control study.
    Source: Smeeth L, Cook C, Fombonne E, Heavey L, Rodrigues LC, Smith PG, Hall AJ. MMR vaccination and pervasive developmental disorders. Lancet 2004;364:963–9.
17. A. The key difference between a cohort and case-control study is that, in a cohort study, subjects are enrolled on the basis of their exposure, whereas in a case-control study subjects are enrolled on the basis of whether they have the disease of interest or not. Both types of studies assess exposure and disease status. While some cohort studies have been conducted over several years, others, particularly those that are outbreak-related, have been conducted in days. Either type of study can be used to study a wide array of health problems, including infectious and non-infectious.
18. A, C, D. A cross-sectional study or survey provides a snapshot of the health of a population, so it assesses prevalence rather than incidence. As a result, it is not as useful as a cohort or case-control study for analytic epidemiology. However, a cross-sectional study can easily measure prevalence of exposures and outcomes.
19. A. The epidemiologic triad of disease causation refers to agent-host-environment.
20. C. Onset of symptoms
    D. Usual time of diagnosis
    A. Exposure
21. A, B, C, D. A reservoir of an infectious agent is the habitat in which an agent normally lives, grows, and multiplies, which may include humans, animals, and the environment.
22. B, C, D. Indirect transmission refers to the transmission of an infectious agent by suspended airborne particles, inanimate objects (vehicles, food, water) or living intermediaries (vectors such as mosquitoes). Droplet spread is generally considered short-distance direct transmission.
23. A, B, D, E. Disease control measures are generally directed at eliminating the reservoir or vector, interrupting transmission, or protecting (but not eliminating!) the host.
24. A. Disease 1: usually 40–50 cases per week; last week, 48 cases
    D. Disease 2: fewer than 10 cases per year; last week, 1 case
    B. Disease 3: usually no more than 2–4 cases per week; last week, 13 cases
25. D. A propagated epidemic is one in which infection spreads from person to person.