Observingthedistributionofdiseaseorhealth related events in human population. • Identify the characteristics with which the disease is associated. • Basically 3 questions are asked who, when and where. • Who means the person affected, where means the place and when is the time distribution.

1. Types of epidemiological studies
• Types of studies Alternative name Unit of study
• OBSERVATIONAL STUDIES
– Descriptive studies
– Analytical studies
• Ecological Correlation Populations
• Cross-sectional Prevalence Individuals
• Case-control Case-reference Individuals
• Cohort Follow-up Individuals
• EXPERIMENTAL STUDIES (Intervention Studies)
– Randomized control trials Clinical Trials Patients
– Field trials Healthy People
– Community trial Community studies Communities

2. Descriptive Epidemiology
• Descriptive study is the first phase of
epidemiological investigation.
• Observing the distribution of disease or health
related events in human population.
• Identify the characteristics with which the
disease is associated.
• Basically 3 questions are asked who, when and
where.
• Who means the person affected, where means
the place and when is the time distribution.

3. Steps of descriptive study
• Defining the population.
• Defining the disease.
• Describing the disease by (a) Time (b) Place
(c) Person
• Measurement of disease.
• Comparing with known indices.
• Formulation of etiological hypothesis.

4. Defining the Population :-
• Age and sex composition of selected group.
• Education
• Occupation
• Socio-economic status
• Marital status

5. Defining the disease:-
Mainly operational definition of the disease keeping
in the mind the objective of study.
Describing the disease :-
Time distribution
Short term flactuation Periodic Fluctuation Long term fluctuation
A. Common Source epidemic A. Seasonal Trend A. Secular Trend
1. Single exposure or point source B. Cyclic Trend
2. Continuous or multiple source
B. Propagated Epidemic
1. Person to Person
2. Arthropod vector
3. Animal reservoir
C. Slow or Modern Epidemic

6. • Short term fluctuation
Common Source epidemic- Single exposure or point source
– eg. Food poisoning
Continuous or multiple source – contaminated water-
Cholera
. Propagated Epidemic- Hepatitis A, Polio epidemic
Periodic Fluctuation
Seasonal Trend – measles, varicella, URTI, malaria etc.
Cyclic Trend – measles in prevaccination era appeared in
major peaks every 2-3 years and rubella in every 6-9 years
Long term fluctuation- Secular Trend –progressive increase
or decrease over a long period of time. CHD, Diabetes
showed an upward trend during past 50 years

7. Place distribution
• Presence of disease varies in different geographical
areas depended upon the environmental condition
and genetic variation of the host.
a. International variation – Ca Cx and Ca oral cavity
in India, Ca breast in western countries.
b. National variation – malaria, endemic goitre,
flurosis
c. Rural urban variation –urban- lung Ca, CVDs,
mental illnesses, chr. Bronchitis. Rural- skin
diseases, zoonosis, soil transmitted dis.s
d. Local distribution – endemic goitre, yellow fever

8. Person distribution
Age : Childhood – measles, Upper respiratory illness,
Pneumonia etc.
Middle Age :- Cancer, Accident, Occupational diseases,
Peptic ulcer
Old Age :- Atherosclerosis, Cancer, Cardiovasculardiseases,
Hypertension, Chronic Degenerative diseases.
Bio Modality – Hodgekin’s disease
Sex : Some diseases are common in females and some common
in males. In males - lung cancer
In females Breast, Ovarian, Cervical cancer
Marital Status : Cancer cervix more common in early marriage,
multiple sex partner.
Occupation : sedentary occupation more of cardiovascular risk,
diabetes, obesity.
Occupational hazards like skin cancer and allergy in dye
industry, Bronchitis and lung disease in dusty trades .

9. Social class : Low socio-economic status:
Malnutrition, Respiratory and skin diseases
High socio-economic status : cardiovascular
diseases, coronary artery diseases.
Behaviour : Human behaviour is looked as a risk
factor. Coronary heart disease, cancer, obesity,
accidents are modern day diseases.
Smoking, sedentary life style, overeating, drug
abuse
Stress : effect on susceptibility, exacerbation of
symptoms, compliance of medical regimen.

10. Measurement of disease
• Mandatory to know the disease load in the population.
Measured in terms of mortality, morbidity and dissability.
• Mortality can be measured as crude rates or specific rates.
e.g. Age specific and sex specific mortality rates.
• Morbidity measured in terms of incidence rates and
prevalence rates (point prevalence and period prevalence).
Incidence rate obtained by longitudinal studies and
prevalence rate by cross sectional studies (simplest form of
observational studies based on single examination at one
point of time in the whole population).
• Longitudinal studies are done for a longer time and follow
up examination done useful for study of natural history of
disease, find out risk factors and incidence rate of diseases.

11. Comparing with known-indices :
The epidemiological studies done to compare
with known indices or comparing with two
communities or two groups of population.
Formulation of hypothesis :
1. The population characteristics
2. Specific causes
3. Expected outcome of the disease
4. Dose response relationship
5. Time response relationship

12. Types of descriptive study
1. Case report and case series :
Describe about the case, sign, symptoms,
laboratory test, finding etc.
2. Cross sectional descriptive studies
3. Longitudinal descriptive studies.
Epidemiological descriptive studies done
according to person, place and time.

13. Difference between Descriptive and Analytical Epidemiology
Descriptive Epidemiology Analytical Epidemiology
Only one group studies At least two groups are studied for
comparison
At the start of study there is no
explicit hypothesis regarding cause
effect relationship
At the start of the study there is definite
hypothesis regarding an exposure
possibly causing an outcome.
The study ends in development of
possible hypothesis regarding cause
and effect relationship but does not
confirm or reject such hypothesis
At the end of the study it confirms or
rejects the hypothesis with which it
started