Epidiomiology approaches

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Epidemiology Approaches

Epidemiology is the study of the distribution and determinants of health-related states in specified populations, and the application of that study to the control of health problems. There are three major approaches:

1. Descriptive Epidemiology

Descriptive studies are usually the first phase of an epidemiological investigation. They observe the distribution of disease in populations and identify characteristics associated with the disease. They ask three fundamental questions:

When is the disease occurring? - time distribution
Where is it occurring? - place distribution
Who is getting the disease? - person distribution

Procedures in descriptive studies (Park's Table 8):

Defining the population to be studied
Defining the disease under study
Describing the disease by time, place, and person
Measurement of disease
Comparing with known indices
Formulation of an aetiological hypothesis

The "defined population" can be the whole population of a geographic area, or a representative sample. At the end of a descriptive study, you formulate a hypothesis - but you do not test it. That is the job of analytical studies.

Types of descriptive studies:

Case reports / case series - observations about individual patients or small groups
Cross-sectional studies - prevalence surveys; exposure and disease measured simultaneously in the population at a single point in time
Ecological studies - population-level data are used (not individual data); unit of analysis is a group, not a person

2. Analytical Epidemiology

Analytical studies are the second major type. In contrast to descriptive studies that look at entire populations, analytical studies focus on the individual within the population. The object is not to formulate, but to test hypotheses. They determine:

Whether or not a statistical association exists between a disease and a suspected factor
If one exists, the strength of the association

Analytical studies comprise two distinct types of observational studies:

A. Case-Control Study (Retrospective)

Case-control studies are often called "retrospective studies" and are a common first approach to test causal hypotheses. Three distinct features:

Both exposure and outcome (disease) have already occurred before the start of the study
The study proceeds backwards from effect to cause
It uses a control or comparison group

The investigator starts with cases (people with disease) and controls (people without disease), then looks back in time to compare their exposures.

Direction of inquiry: present → past

Measure of association: Odds Ratio (OR)

Basic steps:

Selection of cases and controls
Matching
Measurement of exposure
Analysis and interpretation (2×2 contingency table)

Advantages: Quick, cheap, good for rare diseases and diseases with long latency; can study multiple exposures.

Disadvantages: Susceptible to recall bias, selection bias; cannot calculate incidence; cannot study rare exposures.

B. Cohort Study (Prospective)

The cohort study starts with a group of disease-free individuals, classifies them by exposure status, and follows them over time to see who develops the disease.

Direction of inquiry: past/present → future

Comparison groups in cohort studies:

Internal comparisons - single cohort classified by degree of exposure (e.g., smokers by number of cigarettes/day)
External comparisons - separate exposed vs. unexposed cohort (e.g., radiologists vs. ophthalmologists)
Comparison with general population rates - the exposed group's experience compared with general population rates in the same area

Measure of association: Relative Risk (RR) / Incidence Rate Ratio

Advantages: Can calculate incidence and relative risk; temporal sequence of exposure → disease is clear; can study multiple outcomes from one exposure; less susceptible to bias.

Disadvantages: Expensive, time-consuming; requires large numbers; loss to follow-up is a major problem.

Feature	Case-Control	Cohort
Starting point	Disease (cases vs. controls)	Exposure status
Direction	Retrospective	Prospective (usually)
Measure	Odds Ratio	Relative Risk
Best for	Rare diseases	Rare exposures
Cost	Low	High

3. Experimental (Interventional) Epidemiology

Experimental studies are similar in approach to cohort studies, except the conditions are under direct control of the investigator. They involve deliberate application or withdrawal of a suspected cause or intervention in the experimental group, while making no change in the control group, then comparing outcomes.

This contrasts sharply with observational studies (descriptive, case-control, cohort) where the epidemiologist only observes the natural course of events.

Aims:

To provide "scientific proof" of aetiological or risk factors
To measure the effectiveness and efficiency of health services for prevention, control, and treatment of disease

Types:

Type	Description
Randomized Controlled Trial (RCT)	Gold standard; participants randomly allocated to intervention vs. control; minimizes confounding
Field trial	Intervention applied in the community to healthy people at risk (e.g., vaccine trials)
Community trial	Entire communities (not individuals) randomized to intervention
Animal experiments	Used to confirm aetiological hypotheses, test vaccines/drugs, and study pathogenesis

Additional challenges compared to observational studies: cost, ethics, and feasibility.

Summary: The Epidemiological Hierarchy

Experimental Studies (RCTs)           ← Highest causal evidence
    ↑
Analytical Studies (Cohort, Case-Control)
    ↑
Descriptive Studies (Cross-sectional, Ecological, Case reports)
    ↑
Clinical impression / anecdote        ← Lowest evidence

The typical sequence in epidemiological research is:

Descriptive study → generates hypothesis
Analytical study → tests hypothesis
Experimental study → confirms causation and tests interventions

Park's Textbook of Preventive and Social Medicine, pp. 83-93

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