Classification of epidemiology method, descriptive epidemiology
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Classification of Epidemiological Methods & Descriptive Epidemiology
(Source: Park's Textbook of Preventive and Social Medicine)
Classification of Epidemiological Studies
Epidemiological studies are broadly classified into two major categories:
1. Observational Studies
The investigator measures but does not intervene - nature takes its own course.
a. Descriptive Studies
Concerned with describing the distribution of disease in populations by time, place, and person. They form the first phase of epidemiological investigation and generate hypotheses.
b. Analytical Studies
Go further by analyzing the relationship between health status and other variables. The unit of study is the individual. They test hypotheses generated by descriptive studies.
| Type | Also Called | Unit of Study |
|---|---|---|
| Ecological | Correlational | Populations |
| Cross-sectional | Prevalence study | Individuals |
| Case-control | Case-reference | Individuals |
| Cohort | Follow-up study | Individuals |
2. Experimental Studies (Intervention Studies)
Involve an active attempt to change a disease determinant or the progress of disease. Subject to extra constraints since participants' health may be at stake.
| Type | Also Called | Unit of Study |
|---|---|---|
| Randomized Controlled Trials | Clinical trials | Patients |
| Field trials | - | Healthy people |
| Community trials | Community intervention studies | Communities |
These study types are not watertight compartments - they complement one another.
Descriptive Epidemiology - In Detail
Definition and Purpose
Descriptive studies are usually the first phase of any epidemiological investigation. They are concerned with observing the distribution of disease or health-related characteristics in human populations and identifying characteristics with which the disease seems to be associated.
The three fundamental questions asked are:
| Question | Variable |
|---|---|
| When is the disease occurring? | Time distribution |
| Where is it occurring? | Place distribution |
| Who is getting the disease? | Person distribution |
Procedures in Descriptive Studies (Table 8, Park's)
- Defining the population to be studied - The "population base," including its composition by age, sex, occupation, cultural characters, etc.
- Defining the disease under study - The epidemiologist requires an "operational definition" - one that can be measured in the community with accuracy. Unlike clinical definitions, operational definitions spell out clear, measurable criteria.
- Describing the disease by:
- a. Time
- b. Place
- c. Person
- Measurement of disease - Calculating appropriate rates and ratios
- Comparing with known indices - Benchmarking findings
- Formulation of an aetiological hypothesis - The ultimate product of descriptive work
Describing Disease by TIME
Time distribution has three components:
I. Short-term fluctuations
The best-known short-term fluctuation is an epidemic - defined as "the occurrence in a community or region of cases of an illness or other health-related events clearly in excess of normal expectancy." Epidemicity is relative to the usual frequency in the same area, same population, and same season.
Types of Epidemics:
A. Common-source epidemics
- (a) Point-source (single exposure): Exposure is brief and essentially simultaneous; all cases develop within one incubation period. The epidemic curve has one peak. Example: food poisoning outbreak.
- (b) Continuous or multiple exposure: Prolonged exposure from the same source.
B. Propagated epidemics
- (a) Person-to-person
- (b) Arthropod vector
- (c) Animal reservoir
C. Slow (modern) epidemics - e.g., obesity, cardiovascular disease
II. Periodic fluctuations
Seasonal and cyclical patterns of disease.
III. Long-term (secular) trends
Changes in disease frequency over decades.
Describing Disease by PLACE
Distribution of disease in geographic areas - helps identify environmental, climatic, or socioeconomic factors. Includes international comparisons, urban/rural differences, and local clustering.
Describing Disease by PERSON
Common characteristics examined include:
- Age
- Sex
- Ethnic group / race
- Marital status
- Occupation
- Social class / socioeconomic status
- Religion
- Lifestyle habits (diet, smoking, alcohol)
- Migration status
Important note: Grouping by time, place, and person is an initial classification of variables and is NOT a classification of causal factors.
Uses of Descriptive Epidemiology
- Magnitude of disease load - Provides morbidity/mortality rates and ratios in the community
- Clues to disease aetiology - Identifies groups at higher risk, leading to hypothesis generation. If a disease is more frequent in a particular group, hypotheses are formulated to explain the increased frequency.
- Planning health services - Provides background data for planning, organizing, and evaluating preventive and curative services
- Research foundation - Describes variations in disease occurrence by time, place, and person; contributes data for further research
Descriptive vs. Analytical Epidemiology
| Feature | Descriptive | Analytical |
|---|---|---|
| Unit of interest | Populations | Individuals |
| Objective | Formulate a hypothesis | Test a hypothesis |
| Phase | First (exploratory) | Second (confirmatory) |
| Question asked | Who, when, where? | Why? How? |
| Association sought | Possible | Confirmed statistically |
- Park's Textbook of Preventive and Social Medicine, pp. 75-77 (Epidemiological Methods & Descriptive Epidemiology sections)
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