classification of epidemiological methods, descriptive epidemiology

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Classification of Epidemiological Methods and Descriptive Epidemiology

Source: Park's Textbook of Preventive and Social Medicine

Classification of Epidemiological Methods

The primary concern of the epidemiologist is to study disease occurrence in people, who are exposed during their lives to numerous factors and circumstances that may have a role in disease aetiology. Epidemiological studies are classified into two broad categories:

EPIDEMIOLOGICAL STUDIES
├── 1. OBSERVATIONAL STUDIES
│   ├── a. Descriptive Studies
│   └── b. Analytical Studies
│       ├── (i)  Ecological / Correlational     → populations as unit of study
│       ├── (ii) Cross-sectional / Prevalence   → individuals as unit of study
│       ├── (iii) Case-control / Case-reference → individuals as unit of study
│       └── (iv) Cohort / Follow-up             → individuals as unit of study
│
└── 2. EXPERIMENTAL STUDIES (Intervention Studies)
    ├── a. Randomized Controlled Trials / Clinical Trials → patients as unit of study
    ├── b. Field Trials                                   → healthy people as unit of study
    └── c. Community Trials / Community Intervention      → communities as unit of study

Key distinctions:

Observational studies allow nature to take its own course; the investigator measures but does not intervene.
Descriptive studies are limited to describing the occurrence of disease in a population.
Analytical studies go further by analyzing the relationship between health status and other variables.
Experimental/Intervention studies involve an active attempt to change a disease determinant or the progress of a disease; they are subject to extra constraints since participants' health is at stake.

These study types are not watertight compartments - they complement one another.

Descriptive Epidemiology

Descriptive studies are usually the first phase of an epidemiological investigation. They are concerned with observing the distribution of disease or health-related characteristics in human populations and identifying characteristics associated with the disease.

The three fundamental questions asked are:

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)

Step	Procedure
1	Defining the population to be studied
2	Defining the disease under study
3	Describing the disease by (a) time, (b) place, (c) person
4	Measurement of disease
5	Comparing with known indices
6	Formulation of an aetiological hypothesis

A. Time Distribution

Time is one of the most important variables in descriptive epidemiology. Time trends in disease occurrence can be classified into three types:

I. Short-term Fluctuations (Epidemics)

An epidemic is 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, population, and season.

Types of epidemics:

A. Common-source epidemics

(a) Single exposure / Point-source epidemic - Exposure is brief and simultaneous; all cases develop within one incubation period (e.g., food poisoning). The epidemic curve shows one peak. The "median incubation period" is the time for 50% of cases to appear after exposure.
(b) Continuous or multiple exposure epidemics - Exposure is prolonged or repeated from the same source.

B. Propagated epidemics

(a) Person-to-person - Transmission continues until susceptibles are depleted; speed depends on herd immunity and secondary attack rate. Cases show sequential peaks separated by one incubation period.
(b) Arthropod vector
(c) Animal reservoir

C. Slow (modern) epidemics - e.g., cardiovascular disease, cancer - unfolding over decades.

A graph of the time distribution of epidemic cases is called the epidemic curve (Fig. 4). It may suggest: (1) a time relationship with exposure to a suspected source, (2) a cyclical or seasonal pattern, and (3) whether spread is common-source or propagated.

II. Periodic Fluctuations

(i) Seasonal trend - Well-known for many communicable diseases (measles peaks in early spring, bacterial GI infections in summer, respiratory infections in winter). May relate to temperature, humidity, rainfall, vectors, or overcrowding. Example: Dengue in India peaks in September-November coinciding with late summer and rains.

(ii) Cyclic trend - Recurring rises and falls over periods of several years due to buildup and depletion of susceptibles in the population (e.g., measles shows cycles of 2-3 years; cholera pandemic cycles).

III. Long-term / Secular Trends

Changes in disease frequency over many years or decades, reflecting changes in host resistance, virulence, living standards, or treatment (e.g., the long-term decline in tuberculosis mortality before effective chemotherapy was introduced).

B. Place Distribution

Study of the geographical distribution of disease. This includes:

Spot maps - Marking cases on a map (John Snow's classic cholera map of Broad Street pump, 1854)
Geographical variation - International, national, rural-urban differences
Local distribution - Within a community or institution
Natural foci of disease - Areas where vectors and reservoirs exist

Place distribution helps identify environmental factors, possible sources of infection, and high-risk areas.

C. Person Distribution

The study of who gets the disease - examining characteristics of the persons affected. Key variables include:

Personal Variable	Examples/Notes
Age	Single most important variable; most disease rates vary with age
Sex	Biological and behavioral differences affect risk
Ethnic group / Race	Genetic, cultural, socioeconomic influences
Occupation	Occupational exposure to hazards
Religion and customs	Dietary habits, circumcision, etc.
Social class	Socioeconomic status correlates with disease patterns
Marital status	Psychological and lifestyle factors
Family clustering	Genetic susceptibility or shared exposure
Migration	Changing risk when moving between environments

Formulation of an Aetiological Hypothesis

A key output of descriptive epidemiology is the formulation of a hypothesis linking a suspected factor to a disease. A good hypothesis must be:

Specific - specifying the exposure, population, dose, duration, and outcome
Testable

Poor hypothesis: "Cigarette smoking causes lung cancer."
Improved: "The smoking of 30-40 cigarettes per day causes lung cancer in 10% of smokers after 20 years of exposure."

Uses of Descriptive Epidemiology

Provides data on the magnitude of disease burden in the community (morbidity and mortality rates).
Provides clues to disease aetiology and helps formulate aetiological hypotheses - the existence of a possible causal association is usually first recognized in descriptive studies.
Provides background data for planning, organizing, and evaluating preventive and curative services.
Contributes to research by describing variations in disease occurrence by time, place, and person.

Transition to Analytical Epidemiology

After descriptive studies generate a hypothesis, analytical studies test it. While descriptive studies look at entire populations, analytical studies evaluate the individual within the population. The two main types are:

Case-control studies - Start with cases (disease present) and controls (disease absent), then look back at exposure.
Cohort studies - Start with exposed and unexposed groups, then follow forward to see who develops disease.

Both determine: (a) whether a statistical association exists between a disease and a suspected factor, and (b) the strength of that association.

Source: Park's Textbook of Preventive and Social Medicine, Chapter on Epidemiology (pp. 74-77 approx.)

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