Epidemiological Study Designs & Risk Measures

Descriptive studies

Who, What, When, Where

Use: GENERATING hypothesis

Ecological, Cross-Sectional

Analytical studies

How, Why

Use: TESTING hypothesis

Case-Control (OR), Cohort (RR)

Ecological Study

Group level data

Advantages: quick, inexpensive

Disadvantages: ecological fallacy

Ecological Fallacy

Assuming something true about a GROUP is also true about every INDIVIDUAL in that group

Cross-Sectional Study

Measures disease and exposure at the SAME TIME

• Pros: Quick, measures PREVALENCE

• Cons: Cannot measure incidence or determine what came first

Case-Control Study

Starts with people who HAVE and DON'T HAVE a disease

• Pros: Quick, cheap, good for RARE diseases

• Cons: Hard to know if exposure came before disease; cases and controls may not represent the population.

What are cases and controls?

• Cases = HAVE disease

• Controls = NO disease

What does a case-control study calculate?

• Odds Ratio (OR)

• Cannot directly calculate incidence or prevalence

Case control studies: Odds Ratio (OR)

How do you interpret OR?

• OR = 1 → No association

• OR > 1 → Higher odds (risk factor)

• OR < 1 → Lower odds (protective factor)

How do you explain OR in a sentence?

People WITH the disease had ___ times the odds of being EXPOSED compared to those WITHOUT the disease

What is a cohort study?

• Starts with people who are EXPOSED or NOT exposed.

• Follows them over TIME to see who gets sick.

What does a cohort study calculate?

• Relative Risk (RR)

• Can calculate incidence and prevalence.

Pros and cons of cohort studies?

• Pros: Can tell if exposure happened BEFORE disease.

• Cons: Long, expensive, people may drop out.

Related Rates formula

How do you interpret RR?

• RR = 1 → No association

• RR > 1 → Higher risk (risk factor)

• RR < 1 → Lower risk (protective factor)

How do you explain RR in a sentence?

People who were EXPOSED were ___ times as likely to develop the DISEASE compared to those NOT exposed

Case-Control vs. Cohort?

Case-Control	Cohort
Disease FIRST	Exposure FIRST
Looks BACK	Follows FORWARD
OR	RR
Rare diseases	Exposure before disease

How do you identify each study on an exam?

• Ecological: Comparing GROUPS

• Cross-Sectional: Survey at ONE time

• Case-Control: Starts with DISEASE, looks BACK

• Cohort: Starts with EXPOSURE, follows over TIME

Which study measures PREVALENCE?

Cross-Sectional = SNAPSHOT (existing cases)

Which study measures BOTH prevalence and incidence?

Cohort = BOTH (existing + new cases)

Which study measures NEITHER?

Case-Control = NEITHER (starts with sick vs. healthy)