Introduction to epidemiology

Yu Cheng Hsu

Learning objectives

• Define epidemiology
• Describe basic terminology and concepts of epidemiology
• Describe epidemiological study designs

What is epidemiology?

Definition

Study of the distribution and determinants of health-related states among specified populations and the application of that study to the control of health problems.

Some common questions that epidemiologists ask include:

• Who gets sick?
• When do they get sick?
• Where do they get sick?
• How do they get sick?
• Why do they get sick?
• How can we prevent them from getting sick?

Epidemiology

Epidemiological tools

Based on these questions we can categorize epidemiological tools into three main types:

• Descriptive epidemiology: Describes the distribution of disease in a population by person, place, and time.

• Analytic epidemiology: Identifies the determinants of disease by testing hypotheses about the relationship between exposures and outcomes.

• Experimental epidemiology: Tests the efficacy of interventions to prevent or treat disease by manipulating the exposure of interest and observing the outcome.

Describe a disease

JHU COVID-19 dashboard

• What kind of disease information can we get?

Getting disease descriptions

• Case study
  • Common tools: Interview with case
  • Purpose: In depth investigate the disease process and identify potential causes and risk factors.
• Cross-sectional study
  • Common tools: Survey (one single time)
  • Purpose: Assess the prevalence of disease and its association with potential risk factors at certain time.
• Ecological study
  • Common tools: health records, government statisticss
  • Purpose: Population level to assess the trend, association of a disease.

Understand association

• Cohort study
  • Common tools: health records, participant recruit (optional)
  • Perspective: Control on the exposure and test the outcome is developed
• Case-control study
  • Common tools: health records
  • Perspective: Control on the outcome and test what exposure is associated with the outcome

Cohort study

Cohort

A group of people who share a common characteristic or experience within a defined period of time.

Idea

1. Select a group of people (cohort)
   1. with shared similar characteristics (Age, sex, etc.)
   2. with different exposures (e.g., smoking vs. non-smoking)
2. Follow them over time to see who develops the disease of interest (e.g., lung cancer).

Restrospective cohort study

Looking back from the history of the cohort to determine exposure status and outcome.

1. Find out who get exposured and who do not get exposed in the past.
2. Find out who develops the disease of interest after the exposure.

Illustration of retrospective cohort study

Prospective cohort study

Looking back from the history of the cohort to determine exposure status and looking forward to track outcome.

1. Find out who get exposured and who do not get exposed in the past.
2. Find out who develops the disease of interest after the exposure. (Usually after the study starts)

Illustration of prospective cohort study

Case control study

1. we select a group of people with
   • similar characteristics (e.g., age, sex)
   • with different disease (e.g., cancer vs. non-cancer),
2. Check what kind of exposure they had before the disease

Illustration of case-control study

Comparison

Feature	Cohort	Case-Control
Starting Point	Exposure status	Disease status (Outcome)
Direction of Inquiry	Forward (Exposure → Outcome)	Backward (Outcome → Exposure)
Primary Metric	Relative Risk (RR)	Odds Ratio (OR)
Ideal Use Case	Rare exposures	Rare diseases
Can measure incidence?	Yes	No

Experimental epidemiology

Randomized controlled trial (RCT)

Idea

1. Select a group of people with similar characteristics (e.g., age, sex)
2. Randomly assign them to treatment (e.g., drug) or control (e.g.,placebo, reference point),
3. Follow-up to see improvement in health outcomes to test the efficacy of the treatment.

Illustration of RCT

Evidence level of different tools

1. RCTs
2. Cohort studies
3. Case-control Studies
4. Case study

• Higher-level evidence in terms of
  • Ability to establish causality
  • Higher control for confounding factors
  • Less prone to bias
• Lower-level evidence
  • Cheaper and easier to conduct
  • Useful for generating hypotheses and exploring rare diseases
  • Less ethical concerns

Wrap-up

• Define epidemiology

• Describe basic terminology and concepts of epidemiology

• Describe epidemiological study designs

• What’s next?

  • We are doing code and quantitative analysis for the next class