epidemiology- quiz 2

Confounding

• Definition: A confounder is a third variable that distorts the observed relationship between the independent variable (exposure) and the dependent variable (outcome).

• Example: If you're studying the relationship between physical activity and heart disease, age might be a confounder because it’s associated with both activity levels and heart disease risk.

Confounding

• Key Characteristics:

  • Must be associated with both exposure and outcome.

  • Must not be on the causal pathway.

Goal: Control for it (e.g., through stratification, regression, or randomization)

Effect Modification (Interaction)

• Definition: Occurs when the effect of the main exposure on the outcome differs depending on the level of a third variable.

• Example: The effect of physical activity on reducing heart disease might be stronger in older adults than in younger adults.

Effect Modification (Interaction)

• Key Characteristics:

  • Not a nuisance—it's a finding of interest.

  • Report it rather than control for it.

Cross-sectional

Data collected at one point in time

Quick, inexpensive

No causality

Case-control

Compares people with outcome (cases) to those without (controls), looks back for exposure

Good for rare diseases

Recall bias, no causality

Cohort (Prospective)

Follows exposed and unexposed groups over time to see who develops outcome

Strong for temporality

Time-consuming, expensive

Randomized Controlled Trial (RCT)

Participants randomly assigned to exposure/intervention or control

Best for causality

Costly, may have ethical limits

Ecological

Uses population-level data

Good for hypothesis generation

Ecological fallacy risk

RCT

Random assignment mentioned?

Cohort

Tracking over time?

Case- control

Looking back after outcome?

Cross-sectional

Snapshot in time?

Frequency, Intensity, Time, Type

FITT

Frequency

How often (e.g., days/week)

Intensity

How hard (absolute or relative)

Time

Duration per session

Type

Mode of activity

Absolute intensity

Fixed value

METs, kcal/min, mph

Relative intensity

Individual capacity

%HRmax, RPE, %VO₂max

Subjective Measures

• Examples: Questionnaires, logs, interviews

• Pros:

  • Low cost

  • Feasible for large populations

• Cons:

  • Recall bias

  • Social desirability bias

  • Less precise

Objective Measures

• Examples:

  • Doubly labeled water: Gold standard for energy expenditure; very accurate, but expensive and complex.

  • Indirect calorimetry: Measures gas exchange; used in lab settings.

  • Heart rate monitors: Reasonable estimate of intensity, influenced by non-exercise factors.

  • Accelerometers: Measures movement; good for patterns and intensity.

Objective Measures

• Advantages:

  • Objective

  • Captures frequency, intensity, and duration

• Disadvantages:

  • Can miss upper body movement or cycling

  • Doesn’t capture context (type of activity)

  • Expensive for large populations

Number Needed to Treat

[a / (a + b)] - [c / (c + d)]