Epidemiology Practice Flashcards

Question 1

What is epidemiology in the context of public health?

Accepted Answer

Epidemiology is the study of the distribution and determinants of health-related events in specified populations, and the application of that study to control health problems. Distribution refers to who is affected, where, and when, while determinants are the factors that cause or influence those events. It focuses on populations rather than individual patients.

Question 2

What does the term population mean in epidemiology?

Accepted Answer

In epidemiology, a population is a defined group of people who share a characteristic, such as living in the same city, attending the same school, or belonging to the same age range. Defining the population clearly matters because it sets the denominator for rates and determines who the findings apply to.

Question 3

What is a risk factor, and how does it differ from a cause?

Accepted Answer

A risk factor is any characteristic, exposure, or behavior associated with a higher probability of a health outcome. Association alone does not prove causation, so a risk factor may be a true cause or a marker of one. A cause, by contrast, is a factor whose removal or change would alter the outcome.

Question 4

What do endemic and epidemic mean in epidemiology?

Accepted Answer

Endemic describes a condition that is constantly present at a predictable level within a population or region. Epidemic describes a level of occurrence clearly above what is expected for that population and place, over a given period. The threshold for calling something epidemic depends on the baseline expected count, not on a fixed number.

Question 5

What is a pandemic, and how does it differ from an epidemic?

Accepted Answer

A pandemic is an epidemic that spreads across multiple countries or continents and affects many people. The difference is one of geographic scope rather than severity. An epidemic may be confined to a single town, while a pandemic reaches many regions at once.

Question 6

What is a case definition and why is it needed?

Accepted Answer

A case definition is a standard set of criteria, such as clinical features, time, place, and person characteristics, used to decide whether an individual counts as a case. It is needed so that everyone counting cases uses the same rules, which keeps data comparable across clinics, areas, and time.

Question 7

What is the difference between morbidity and mortality?

Accepted Answer

Morbidity refers to the presence of disease or illness in a population, including how often it occurs and how it affects daily functioning. Mortality refers to deaths in a population, often expressed as a rate. A condition can have high morbidity with low mortality, so the two measures should not be confused.

Question 8

What does it mean when cases are described as sporadic or clustered?

Accepted Answer

Sporadic cases occur at irregular intervals with no obvious link to one another, reflecting background occurrence of a condition. Clustered cases appear grouped in time, place, or among people with shared exposures, which may signal a common source. Recognizing clustering is an early step in outbreak detection.

Question 9

What is the difference between incidence and prevalence?

Accepted Answer

Incidence counts new cases of a condition that develop during a defined time period among people at risk. Prevalence counts all existing cases, both new and old, at one point in time or across a period. Incidence measures how quickly disease arises, while prevalence reflects how much disease is present in a population.

Question 10

What is a rate, and why do epidemiologists prefer rates over raw counts?

Accepted Answer

A rate expresses the number of events relative to the size of the population at risk, often per a standard multiplier such as 100,000 person-years or per 1,000 people. Raw counts can mislead because a larger city will naturally report more cases. Rates allow fair comparison between groups, places, and time periods of different sizes.

Question 11

What is a denominator in an epidemiologic measure?

Accepted Answer

The denominator is the base quantity in a ratio or rate, such as the total population, the population at risk, or the total person-time observed. For example, cases divided by the total population gives a risk, while cases divided by person-time gives an incidence rate.

Question 12

What is person-time, and when is it used?

Accepted Answer

Person-time is the total time each member of a population is observed and at risk, summed across all participants, often measured in person-years. It is used when people join or leave a study at different times, or when follow-up lengths vary. Using person-time produces an incidence rate that accounts for unequal observation periods.

Question 13

Why can prevalence be misleading when studying causes of disease?

Accepted Answer

Prevalence mixes new cases with existing cases, and existing cases may last a long time because of survival or recovery patterns. A factor that prolongs survival can appear to be associated with higher prevalence even if it does not cause disease. This is why incidence is generally preferred when investigating causes.

Question 14

What is an attack rate and where is it commonly used?

Accepted Answer

An attack rate is the proportion of a group at risk who become ill during a defined period, often during an outbreak among people with a shared exposure. It is calculated as the number of ill people divided by the number exposed. It is commonly used in foodborne and household outbreak investigations.

Question 15

What is relative risk, and how is it interpreted?

Accepted Answer

Relative risk is the ratio of the risk of disease in an exposed group to the risk in an unexposed group. A value of 1 indicates no association, values above 1 suggest higher risk with exposure, and values below 1 suggest lower risk. It is usually calculated in cohort studies.

Question 16

What is an odds ratio and when is it used instead of relative risk?

Accepted Answer

An odds ratio compares the odds of an outcome between exposed and unexposed groups, where odds are cases divided by noncases. It is used in case-control studies, where risk cannot be estimated directly because participants are selected by outcome. When the outcome is rare, the odds ratio closely approximates the relative risk.

Question 17

What is a cross-sectional study and what can it show?

Accepted Answer

A cross-sectional study measures exposures and outcomes in a population at one point in time, like a snapshot. It can show how common a condition is and whether it is associated with characteristics across the group. Because timing of exposure and outcome is unclear, it cannot establish which came first.

Question 18

How does a cohort study identify its participants and outcomes?

Accepted Answer

A cohort study identifies groups defined by exposure status, then follows them forward over time to see who develops the outcome. It is the design that can directly calculate incidence and relative risk. Cohorts may be prospective, enrolling people now and following them, or retrospective, using existing records.

Question 19

How does a case-control study choose its participants and what does it compare?

Accepted Answer

A case-control study selects people with the outcome, called cases, and a comparison group without it, called controls. It then looks backward to compare how often each group had the exposure. It is efficient for rare outcomes and long latency periods, but it cannot directly measure incidence in the population.

Question 20

What is a randomized controlled trial and why is randomization important?

Accepted Answer

A randomized controlled trial assigns participants by chance to an intervention group or a comparison group, then follows both groups for outcomes. Random assignment tends to balance known and unknown confounding factors across groups, so differences in outcomes can be attributed to the intervention. Trials are considered the strongest design for testing interventions.

Question 21

What is an ecological study and what is its main weakness?

Accepted Answer

An ecological study compares groups, such as countries or regions, using summary data rather than data on individuals. It is inexpensive and useful for generating hypotheses about population-level patterns. Its main weakness is the ecological fallacy, which occurs when group-level associations are wrongly assumed to apply to individuals.

Question 22

Why are observational studies used when randomized trials are not possible?

Accepted Answer

Some exposures cannot ethically or practically be assigned to people, such as smoking, occupational hazards, or environmental pollutants. Observational studies examine exposures that people already experience, allowing researchers to study real-world associations. Their results are more vulnerable to confounding and bias, so findings require careful design and interpretation.

Question 23

What is a natural experiment and how does it differ from a trial?

Accepted Answer

A natural experiment is a situation where an exposure is assigned by circumstances outside the researcher's control, such as a policy change or a disaster, creating comparable exposed and unexposed groups. The investigator observes rather than assigns the exposure. Because assignment is not random, confounding remains a concern.

Question 24

What does bias mean in epidemiologic research and why does it matter?

Accepted Answer

Bias is a systematic error that causes study results to differ from the true values, pushing estimates consistently in one direction. Unlike random error, it is not reduced by collecting more participants. It matters because a biased finding can appear precise and still be misleading about who is affected or why.

Question 25

What is confounding and what must a confounder be associated with?

Accepted Answer

Confounding is a distortion of the apparent association between an exposure and an outcome caused by a third factor, the confounder. To be a confounder, a factor must be associated with the exposure, be a cause or risk factor for the outcome, and not lie on the causal pathway between them.

Question 26

What is selection bias and what is one common example?

Accepted Answer

Selection bias occurs when the people included in a study, or retained in it, differ systematically from the target population in ways related to the exposure or outcome. One common example is loss to follow-up, where participants who drop out differ from those who remain. Volunteer bias, where willing participants differ from others, is also common.

Question 27

What is recall bias and when is it most likely to occur?

Accepted Answer

Recall bias is a form of information bias in which participants remember past exposures differently depending on whether they have the outcome. It is most likely in retrospective studies that rely on self-reported history, especially when the outcome is salient or stigmatized. Using records collected before the outcome occurred can reduce this problem.

Question 28

What is the difference between random error and systematic error?

Accepted Answer

Random error is unpredictable variation that changes from one measurement to another and tends to average out as sample size grows. Systematic error is consistent deviation in one direction, which does not average out with larger samples. Random error affects precision, while systematic error affects validity.

Question 29

What is effect modification and how does it differ from confounding?

Accepted Answer

Effect modification occurs when the strength or direction of an association differs across levels of a third factor, which is then called an effect modifier. Unlike confounding, it is a real feature of the data to be described, not an error to be removed. Stratified results are reported separately to reveal it.

Question 30

What are the Bradford Hill considerations used to judge causation?

Accepted Answer

The Bradford Hill considerations are a set of viewpoints, including strength of association, consistency across studies, temporality, dose-response relationship, and biological plausibility, used to weigh evidence for causation. None is a strict test on its own. Temporality, meaning the exposure precedes the outcome, is the only one considered essential.

Question 31

How can investigators reduce confounding in study design or analysis?

Accepted Answer

Investigators can use randomization to balance confounders at baseline, restriction to limit participants to one level of a confounder, and matching to make comparison groups similar. During analysis, stratification and multivariable adjustment can account for measured confounders. Unmeasured confounding cannot be fully removed by these methods.

Question 32

What is public health surveillance and what is its purpose?

Accepted Answer

Public health surveillance is the ongoing, systematic collection, analysis, and interpretation of health data, shared with those who need it to take action. Its purpose is to detect trends, identify outbreaks early, and guide prevention and control programs. Surveillance supports decisions but does not by itself prove what caused a problem.

Question 33

What is the difference between active and passive surveillance?

Accepted Answer

Passive surveillance relies on healthcare providers or laboratories to report cases that come to their attention, which is inexpensive but may miss many cases. Active surveillance involves staff who regularly contact providers or records to find cases, which is more complete but more resource intensive. Both approaches depend on consistent case definitions.

Question 34

Which types of conditions do health agencies require providers to report?

Accepted Answer

Reportable conditions are those that health authorities have legally designated for mandatory notification, typically because they are serious, contagious, or signal a public health threat. The list is set by jurisdiction and changes over time. Reporting allows agencies to detect clusters, track trends, and begin control measures promptly.

Question 35

What are the main steps of a typical outbreak investigation?

Accepted Answer

A typical investigation verifies the outbreak, confirms the diagnosis and case definition, identifies and counts cases, describes them by person, place, and time, and develops hypotheses. Investigators then test those hypotheses with analytic studies, implement control measures, and communicate findings. Steps often overlap, and ongoing surveillance continues throughout.

Question 36

What is an epidemic curve and what does its shape reveal?

Accepted Answer

An epidemic curve is a histogram showing the number of cases by date of onset. A sharp peak followed by a decline often suggests a single brief exposure, while a prolonged plateau or successive waves can suggest continued or repeated exposure. The curve also helps estimate the timing of the source.

Question 37

What is a line listing and what information does it usually contain?

Accepted Answer

A line listing is a spreadsheet in which each row represents one case and each column records a characteristic, such as age, sex, dates of onset and testing, location, and exposures. It allows investigators to sort, compare, and spot patterns quickly. Line listings must be kept confidential because they contain identifying information.

Question 38

What is the difference between a point source and a propagated outbreak?

Accepted Answer

A point source outbreak involves a single common exposure, so cases cluster closely in time and usually occur within one incubation period. A propagated outbreak spreads from person to person or through successive generations of transmission, producing a more prolonged and multi-peaked curve. Identifying which pattern applies guides where investigators look for the source.

Question 39

What is an incubation period and why does it matter in outbreak work?

Accepted Answer

The incubation period is the time between exposure to an agent and the onset of symptoms. Its typical range helps investigators decide how far back to search for exposures and how long to monitor contacts. Because it varies by agent and person, investigators work with typical spans, not fixed values.

Epidemiology Practice Flashcards

1Foundations and Terminology

2Measuring Disease Frequency

3Study Designs in Epidemiology

4Bias, Confounding, and Causation

5Surveillance and Outbreak Investigation

Learning Resources