CBIC Epidemiology 4 — Questions and Answers
Question 1: Which of the following best describes 'confounding' in an epidemiological study of HAIs?
- Misclassification of infection status by the observer
- A third variable that is associated with both the exposure and the outcome (Correct answer)
- Loss of study participants over time
- Systematic error in data collection instruments
Correct answer: A third variable that is associated with both the exposure and the outcome
Confounding occurs when a third variable is independently associated with both the exposure and the outcome, potentially distorting the true relationship between them.
Question 2: In outbreak investigation, a 'propagated epidemic curve' with successive waves of increasing case counts typically indicates which transmission pattern?
- A single common source with a short exposure window
- Continuous exposure to a contaminated common source
- Person-to-person spread with progressive secondary cases (Correct answer)
- A point-source exposure with an extended incubation period
Correct answer: Person-to-person spread with progressive secondary cases
A propagated curve with successive peaks separated by one incubation period indicates person-to-person transmission, where secondary and tertiary cases generate new waves.
Question 3: What is the primary purpose of stratified analysis in HAI epidemiological studies?
- To increase statistical power by combining subgroups
- To identify and control for confounding variables (Correct answer)
- To reduce the sample size needed for significance
- To eliminate selection bias from the study design
Correct answer: To identify and control for confounding variables
Stratified analysis examines the association between exposure and outcome within subgroups defined by potential confounders, allowing assessment and control of confounding effects.
Question 4: A cluster of Legionella cases in a hospital is linked to the cooling tower. Which epidemiological term best describes this type of HAI source?
- Endogenous source
- Common vehicle (Correct answer)
- Propagated source
- Vector-borne source
Correct answer: Common vehicle
A common vehicle refers to a single contaminated source (such as water, food, or equipment) that transmits infection to multiple people, which is the mode for Legionella via cooling towers.
Question 5: Which statistical measure indicates the minimum detectable effect size that a study is powered to identify at a given alpha level?
- Confidence interval width
- P-value threshold
- Sample size
- Statistical power (Correct answer)
Correct answer: Statistical power
Statistical power (typically 80% or higher) is the probability of detecting a true effect of a given magnitude; it determines whether a study is adequately sized to identify meaningful differences.
Question 6: In NHSN HAI surveillance, which denominator is used for calculating the CAUTI rate?
- Patient-days
- Admissions
- Urinary catheter-days (Correct answer)
- Occupied bed-days
Correct answer: Urinary catheter-days
NHSN calculates CAUTI rates per 1,000 urinary catheter-days, which accounts for the device-specific risk exposure time and allows meaningful comparisons across facilities.
Question 7: Which measure of disease frequency accounts for the time each individual is at risk and is expressed per person-time unit?
- Prevalence proportion
- Cumulative incidence
- Incidence density rate (Correct answer)
- Attack rate
Correct answer: Incidence density rate
Incidence density (incidence rate) uses person-time as the denominator, accounting for the varying lengths of time different individuals are observed and at risk.
CBIC Epidemiology 4
Which of the following best describes 'confounding' in an epidemiological study of HAIs?