basic concept of epidemiology
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Basic Concepts of Epidemiology
(Source: Park's Textbook of Preventive and Social Medicine)
1. What is Epidemiology?
The word "epidemiology" is derived from Greek: epi (among) + demos (people) + logos (study). It literally means "the study of what falls upon the people."
Definition (IEA / Last):
"The study of the occurrence and distribution of health-related events, states, and processes in specified populations, including the study of the determinants influencing such processes, and the application of this knowledge to control relevant health problems."
Key components of this definition:
- Study - includes surveillance, observation, hypothesis testing, analytic research, and experiments
- Occurrence and distribution - frequency, patterns by time, place, person
- Health-related events - diseases, disabilities, deaths, and health-related states
- Specified populations - defined groups (not just individuals)
- Determinants - biological, chemical, physical, social, behavioral causes
- Application to control - the ultimate applied goal
2. Historical Milestones
- The Epidemiological Society of London was established in the 1850s
- W.H. Frost became the first professor of epidemiology in the USA (1927)
- Major Greenwood became the first professor of epidemiology and medical statistics at the University of London
- Classical definitions evolved from Parkin (1873) through MacMahon (1960) to modern formulations
3. Three Core Components
According to Park's Textbook of Preventive and Social Medicine, three components are common to all definitions:
a. Disease Frequency
The fundamental measure is a rate or ratio (e.g., prevalence rate, incidence rate, death rate). These allow comparison of disease frequency across populations or subgroups, yielding clues to aetiology.
b. Distribution of Disease
Disease is not uniformly distributed in any population. Epidemiology studies patterns by three axes:
- Person - who is affected? (age, sex, occupation, social class)
- Place - where does it occur? (geographic variation, urban vs. rural)
- Time - when does it occur? (secular trends, seasonal variation, epidemics)
This is the basis of descriptive epidemiology.
c. Determinants of Disease
Epidemiology tests aetiological hypotheses to identify underlying causes (risk factors). This is analytical epidemiology - it explains why patterns occur and informs health programmes and policies.
4. Aims of Epidemiology (IEA)
- To describe the distribution and magnitude of health and disease problems in human populations
- To identify aetiological factors (risk factors) in the pathogenesis of disease
- To provide data for planning, implementing, and evaluating services for prevention, control, and treatment of disease
The ultimate aim is to:
- Eliminate or reduce the health problem or its consequences
- Promote the health and well-being of society as a whole
5. Epidemiological Approach
The epidemiological approach rests on two pillars:
a. Asking Questions
Inspired by Rudyard Kipling's six "honest serving men" - What, Why, When, How, Where, Who - epidemiology asks:
Related to health events:
- What is the problem?
- What is its magnitude?
- Where and when did it happen?
- Who is affected?
- Why did it happen?
Related to health action:
- What can be done to reduce the problem?
- How can it be prevented?
- What resources and organisation are needed?
b. Making Comparisons
The basic approach is to compare groups - those with disease vs. those without, or those exposed to a risk factor vs. those unexposed. Comparisons yield aetiological clues.
Key statistical concepts supporting this:
- Comparability - like must be compared with like (similar age, sex, confounders)
- Randomization - best way to ensure comparability in experimental studies
- Matching - used in case-control and cohort studies when randomization is not possible
- Standardization - for age, sex, or parity comparisons
6. Epidemiology vs. Clinical Medicine
| Feature | Clinical Medicine | Epidemiology |
|---|---|---|
| Unit of study | Individual patient (case) | Defined population (population at-risk) |
| Focus | Diagnosis and treatment | Distribution and determinants of disease |
| Subject matter | "Perceived" (clinical/lab findings) | "Conceptual" (tables, graphs, rates) |
| Direction | Patient comes to doctor | Investigator goes to community |
| Goal | Prognosis + specific treatment | Identify source, mode, aetiology; recommend control |
The two are not antagonistic - they are co-existent and mutually helpful.
7. Basic Measurements in Epidemiology
Epidemiology measures:
- Mortality - death rates (crude, specific, standardised)
- Morbidity - incidence rate, prevalence rate
- Disability - disability rates
- Natality - birth rates, fertility rates
- Health services - utilization, needs, coverage
- Environmental/demographic variables
Key properties of good measurements:
- Validity (measures what it claims to measure)
- Reliability (reproducible)
- Accuracy
- Sensitivity (ability to detect true positives)
- Specificity (ability to exclude true negatives)
8. Types of Epidemiological Studies
| Type | Purpose |
|---|---|
| Descriptive | Describe distribution by person, place, time; generate hypotheses |
| Analytical | Test hypotheses; identify determinants (case-control, cohort studies) |
| Experimental/Interventional | Evaluate interventions; establish causation (RCTs, field trials) |
9. Scope of Modern Epidemiology
Modern epidemiology has expanded far beyond infectious disease to include:
- Chronic disease epidemiology
- Clinical epidemiology
- Cancer epidemiology
- Genetic epidemiology
- Occupational epidemiology
- Psychosocial epidemiology
- Serological epidemiology, and more
Key takeaway: Epidemiology is the quantitative backbone of public health. It links biology to society by measuring how much disease exists (frequency), who gets it and where/when (distribution), and why it occurs (determinants) - with the express goal of action to protect population health.
- Park's Textbook of Preventive and Social Medicine, pp. 64-67
defintion of epidemiology
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Definition of Epidemiology
Etymology
The word comes from Greek:
- Epi = among
- Demos = people
- Logos = study
Literally: "the study of what falls upon the people."
Evolution of Definitions
Definitions have changed as the scope of the subject has expanded:
| Author | Year | Definition |
|---|---|---|
| Parkin | 1873 | "That branch of medical science which treats epidemics" |
| Frost | 1927 | "The science of the mass phenomena of infectious diseases" |
| Greenwood | 1934 | "The study of disease, any disease, as a mass phenomenon" |
| MacMahon | 1960 | "The study of the distribution and determinants of disease frequency in man" |
Current Standard Definition
The most widely accepted modern definition (International Epidemiological Association / Last, 2001):
"The study of the occurrence and distribution of health-related events, states, and processes in specified populations, including the study of the determinants influencing such processes, and the application of this knowledge to control relevant health problems."
Breaking Down the Definition
Each key term carries a specific meaning:
| Term | Meaning |
|---|---|
| Study | Includes surveillance, observation, screening, hypothesis testing, analytic research, experiments, and prediction |
| Occurrence and distribution | Analysis by time, place, and population (person) |
| Health-related events, states & processes | Outbreaks, diseases, disorders, causes of death, behaviours, environmental/socioeconomic processes, effects of preventive programmes, health service use |
| Specified populations | Groups with common contexts and identifiable characteristics (not just individuals) |
| Determinants | Geophysical, biological, behavioural, social, cultural, economic, and political factors that influence health |
| Application to control | The applied aim - to promote, protect, and restore health, and advance scientific knowledge |
Three Core Components Common to All Definitions
- Disease Frequency - measurement of how often disease occurs (rates and ratios)
- Distribution - who gets it, where, and when (descriptive epidemiology)
- Determinants - why it occurs; causal/risk factors (analytical epidemiology)
Key Point
Epidemiology is population-based, not individual-based. The clinician is concerned with the individual patient; the epidemiologist is concerned with disease patterns in the entire population - both the sick and the healthy. As stated in the textbook: "the epidemiologist is interested in the relationship between cases and the population in the form of a rate."
- Park's Textbook of Preventive and Social Medicine, pp. 64-65
uses of epidemiology
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Uses of Epidemiology
According to Morris (1957), epidemiology has seven distinct uses, five of which extend beyond the search for causes and bring it closer to the day-to-day concerns of modern medicine.
1. To Study Historically the Rise and Fall of Disease in the Population
Epidemiology provides a means to study disease profiles and time trends in human populations.
- Health and disease patterns in a community are never constant - they fluctuate over short and long periods of time
- Early epidemiology showed that coronary heart disease, accidents, cancer, and diabetes were all "epidemic" in nature
- As old diseases (e.g., smallpox) are conquered, new ones (e.g., Legionnaires' disease, Lassa fever, AIDS) emerge - epidemiology plays a major role in identifying them
- By studying historical trends, useful projections into the future can be made and emerging health problems identified
2. Community Diagnosis
Epidemiology is described as a "diagnostic tool" of community medicine.
Community diagnosis involves:
- Identification and quantification of health problems in a community in terms of mortality and morbidity rates and ratios
- Identification of correlates - those at risk or in need of health care
- Setting priorities in disease control and prevention
- Establishing a benchmark for future evaluation of health services
- Generating new knowledge about disease distribution, causation, and prevention
- Understanding the social, cultural, and environmental characteristics of the community
3. Planning and Evaluation of Health Services
Epidemiological information provides the fundamental basis for planning health services.
Planning includes:
- Medical care facilities (e.g., number of hospital beds needed for specific diseases, health manpower planning)
- Preventive services (e.g., screening programmes, immunization campaigns, sanitary services)
- Research planning
Evaluation includes:
- Determining whether control or preventive measures are effective in reducing disease frequency
- Measuring cost-effectiveness of interventions (e.g., cost of vaccine, personnel, storage, transport)
- This application of epidemiological principles to health care problems constitutes the "new epidemiology"
4. Evaluation of Individual's Risks and Chances
Epidemiologists calculate and communicate the degree of risk in a population.
- Absolute risk - incidence rates and specific rates
- Relative risk - risk in exposed vs. unexposed groups
- Attributable risk - excess risk due to a particular factor
Classic examples:
- Risk of bearing a child with Down syndrome based on maternal age
- Risk assessment for smokers vs. non-smokers for lung cancer and coronary heart disease (CHD)
5. Syndrome Identification
Epidemiological investigations can be used to define and refine disease syndromes - to correct misconceptions and test clinical observations.
Examples:
- Epidemiological study of peptic ulcer differentiated gastric from duodenal ulcer by showing a "poverty gradient" in gastric ulcer but not in duodenal ulcer
- The Patterson-Kelly syndrome (association between dysphagia and iron-deficiency anaemia) - when tested by epidemiological methods, the association was not confirmed
- Plasma renin studies suggesting that distinct subtypes of essential hypertension may exist
6. Completing the Natural History of Disease
Epidemiology provides the broadest perspective of disease - studying the entire spectrum in the population, not just hospital cases.
- Hospital patients represent only a biased sample (usually the more severe cases)
- The epidemiologist studies all cases in a defined population, regardless of severity or source of care
- Example: Epidemiology revealed that one-third to two-thirds of all deaths due to ischaemic heart disease are sudden (occurring in less than one hour) - most victims never reach hospital. This gave tremendous impetus to the development of intensive coronary care units (ICUs)
- Epidemiological investigations have also yielded large amounts of data on risk factors in chronic diseases
7. Searching for Causes and Risk Factors
This is the core analytical function of epidemiology - relating disease to inter-population differences to identify causes.
Major discoveries through epidemiological research:
- Rubella is the cause of congenital defects in the newborn
- Thalidomide is a teratogenic agent
- Cigarette smoking is a cause of lung cancer
- Oxygen exposure in premature babies causes retrolental fibroplasia (blindness)
For chronic diseases, a single cause is rarely found, which led to the important conceptual shift toward identifying risk factors. The search for causes and risk factors is described as a "ceaseless effort."
Summary Table
| Use | Key Purpose |
|---|---|
| 1. Historical study | Understand time trends; project future health problems |
| 2. Community diagnosis | Identify and quantify health problems in communities |
| 3. Planning and evaluation | Rational allocation of resources; assess effectiveness |
| 4. Individual risk assessment | Calculate absolute, relative, and attributable risk |
| 5. Syndrome identification | Define, refine, or correct disease syndromes |
| 6. Natural history of disease | Complete the full picture beyond hospital cases |
| 7. Causes and risk factors | Identify aetiology through comparative studies |
- Park's Textbook of Preventive and Social Medicine, pp. 104-106
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