K. PARK

1. DESCRIPTIVE
EPIDEMIOLOGY
-Reshma Ann Mathew1

2. CLASSIFICATION OF EPIDEMIOLOGIC
METHODS
Epidemiologic
methods
Observational
studies
Descriptive
studies
Analytical
studies
Ecological
Cross
sectional
Case control
cohort
Experimental or
Interventional
studies
Randomized
controlled
trials
Field trials
Community
trials 2

3. Observational studies allow nature to take its
own course; the investigator measures but
does not intervene.
Descriptive analysis is limited to a description
of the occurrence of a disease in a
population.
3

4. In descriptive epidemiology, it is concerned with
observing the distribution of disease in
human population and identifying the
characteristics with which the disease seems to
be associated
4

5. PROCEDURES IN DESCRIPTIVE STUDIES
1) Defining the population to be studied
2) Defining the disease under study
3) Describing the disease by
4) Measurement of disease
5) Comparing with known indices
6) Formulation of an aetiological hypothesis
a) Time
b) Place
c) Person
5

6. STEP 1: DEFINING THE POPULATION
Descriptive studies are investigations of
population.
A defined population should not only be in terms
of total no., but also in terms of age, sex,
occupation, etc.
6

7. The defined population-
i) could be a whole geographic region or a
representative sample
ii) could be a specially selected group- based on age,
sex, occupation, etc
iii) should be large enough so that it is meaningful
iv) should be stable without migration into or out
v) should not be different from other communities in
the region.
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8. STEP 2: DEFINING THE DISEASE
The epidemiologist defines the disease
which can be measured and identified in
the defined population with a degree of
accuracy.
This is different from the clinician’s
definition of a disease 8

9. STEP 3: DESCRIBING THE DISEASE
Time Place Person
Year,
Season
Month,
Week
Day,
Hour of onset
Duration
Climatic zones
Country,
Region
Urban/Rural
Local community
Towns
Cities
Institutions
Age
Sex
Marital state
Occupation,
Social status,
Education
Birth order
Family size
Height
Weight
Blood pressure,
Blood cholesterol,
Personal habits
9

10. 1) TIME DISTRIBUTION
Time
Distribution
Short term
fluctuations
Common
source
Single/Point
exposure
Continuous /
multiple
exposure
propagated
Person to
person
Arthropod
vector
Animal
reservoir
Slow
(modern)
Periodic
fluctuations
Seasonal
trend
Cyclic
trend
Long term or
secular trends
10

11. EPIDEMIC CURVE
A graph of time distribution of epidemic
cases is called epidemic curve.
11

12. A) SHORT TERM FLUCTUATIONS
An epidemic is the best known short term
fluctuation. It 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”
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13.  Types-
1) Common source epidemics-
i)Single/Point exposure
ii)Continuous/Multiple exposure
2) Propagated epidemics-
i)person to person
ii)arthropod vector
iii)animal reservoir
3) Slow(modern) epidemics
13

14. I) COMMON SOURCE EPIDEMICS
1) Single exposure-
 It can occur due to an infectious agent or as a result of
contamination of the environment and develops within
one incubation period. Eg: Bhopal gas tragedy,
Minamata disease
 The epidemic curve rises and falls rapidly, usually has
one peak
 It tends to be explosive (i.e. clustering of cases within a
short time)
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15. 15

16. 2) Continuous exposure-
It is when the exposure from the same source is
prolonged and the epidemic continues over more
than one incubation period.
The epidemic reaches a sharp peak, but tails off
gradually over a longer period of time.
Eg: A well of contaminated water or nationally
distributed vaccine(polio vaccine) or food; water
borne cholera.
16

17. 17

18. II) PROPAGATED EPIDEMICS
Types- person to person, arthropod, animal
The epidemic shows gradual rise and tails
off over a much longer period of time
It is more likely to occur where there is
i) regular supply of new susceptible
individuals- Births, Immigrants
ii) lowering herd immunity 18

19. 19

20. 20
Course of a typical propagated epidemic

21. B) PERIODIC FLUCTUATIONS
1) Seasonal trend-
 Seasonal variation is characteristic of many
communicable diseases. Eg: Measles,
upper respiratory tract infections(seasonal
rise during winter), Malaria, etc.
 Non-infectious diseases and conditions
may sometimes exhibit seasonal variation.
Eg: Sunstroke, hay fever. 21

22. 22

23. 2) Cyclic trend-
Some diseases occur in cycles spread over short
periods of time (days, weeks, months or years) .
Eg: Influenza pandemics are known to occur at
intervals of 7-10yrs due to antigenic variations.
Non-infectious conditions may also occur in this
trend. Eg: Automobile accidents in the US are
more frequent on weekends.
23

24. C) LONG TERM TRENDS
It refers to changes in the occurrence of
disease over a long period of time.
Eg: Coronary disease, diabetes showing
consistent upward trend and a decline in
TB, polio in developed countries during
the past 50 yrs.
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25. 2) PLACE DISTRIBUTION
Place
variations
1) International
variation
2) National
variation
3) Rural-Urban
variation
4) Local distribution 25

26. A) INTERNATIONAL VARIATION
Descriptive studies have shown that the
pattern of a disease is not the same
everywhere
Eg: Cancer of the stomach is very
common in Japan, but unusual in the US.
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27. B) NATIONAL VARIATION
There are variations in disease occurrence
within countries.
Eg: The distribution of endemic goitre,
fluorosis, malaria, nutritional
deficiencies show variations in their
distribution in India. 27

28. C) RURAL-URBAN VARIATIONS
 Due to differences in population density, levels of
sanitation, deficiencies of medical care, education and
environment factors, there exists a rural-urban variation
 Chronic bronchitis, cardiovascular diseases, accidents
are more frequent in urban than rural areas.
 Skin and zoonotic diseases and soil transmitted
helminths may be more frequent in rural than urban
areas. 28

29. D) LOCAL DISTRIBUTIONS
These variations can be studied with the help of
“spot or shaded” maps. If the map showed
clustering, it may suggest a common source of
infection.
Eg: Study of Cholera epidemic by John Snow in
1854
29

30. STUDY BY JOHN SNOW, 1854
 Spot map of deaths from cholera in Golden Square area,
London, 1854
This pump
was later
suspected and
proved to be a
source of
infection
30

31. MIGRATION STUDIES
The use of migrant studies is a way of
distinguishing genetic and environmental
factors.
Carried out in 2 ways-
1) Study of genetically similar groups but living
under different environmental conditions. Eg:
Twins
2) Study of genetically different groups living in
a similar environment. Eg: Men of Japanese
origin living in USA have higher rate of
coronary heart disease than the Japanese in
Japan
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32. 3) PERSON DISTRIBUTION
The disease can be characterised by defining a
person who develops a disease based on age,
sex, occupation, marital status, social factors,
habits and other host factors.
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33. A) AGE
Certain diseases are more frequent in certain age
groups than others. Eg: Measles in childhood,
cancer in middle age and atherosclerosis in old age.
Many chronic and degenerative diseases show a
progressive increase in prevalence with advancing
age.
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34. BIMODALITY
There may be two separate peaks instead of one in
the age incidence curve of a disease. This is known
as bimodality as seen in Hodgkin’s lymphoma,
breast cancer.
It indicates that there are two different sets of
causative factors even though the clinical and
pathological manifestations of the disease is the
same in all ages. 34

35. 35

36. B) SEX
Variations occur due to-
1) Biologic difference like sex linked genetic
inheritance
2) Cultural and behavioural differences
between the sexes in social settings.
Eg: 4:1 male to female ratio in lung cancer due
to cigarette smoking. 36

37. C) ETHNICITY
Differences in racial and ethnic origin.
Eg: Tuberculosis, sickle cell anemia
37

38. D) MARITAL STATUS
In a study, the mortality rates were lower for
married people than unmarried
It is because according to demographers and
sociologists, marriages are selective with respect
to health of the individual. Healthier the
individual, the more likely to get married.
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39. E) OCCUPATION
Occupation may alter the habit pattern of
employees (Sleep, alcohol, smoking, etc)
Workers in a particular occupation are
exposed to certain types of risk. Eg: Workers in
coal mines are likely to suffer from silicosis
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40. F) SOCIAL CLASS
Health and diseases are NOT equally
distributed in social classes.
Certain diseases show higher prevalence in
upper class (Diabetes, Coronary heart disease,
hypertension)
40

41. G) BEHAVIOUR
Behavioural factors such as smoking, sedentary
life, over-eating, drug abuse lead to certain
diseases (Coronary heart disease, Cancer, etc)
Factors like mass movement (Eg: Pilgrimages)
may also lead to transmission of infectious
diseases.
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42. H) STRESS
 The effects of stress are seen based on the patient’s
response (Susceptibility to disease, Exacerbation of
symptoms, etc)
I) Migration
• Due to migration of people, there is also transmission of
the disease from one place to another.
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43. STEP 4: MEASUREMENT OF DISEASE
 Types-
1) Cross sectional studies-
 Prevalence can be obtained.
 It is based on a single examination of a cross
section of population at one point in time.
 More useful for chronic diseases
43

44. 2) Longitudinal studies-
 Incidence can be obtained.
 The observations are repeated in the same
population over a prolonged period of time
by means of follow up examination.
 Longitudinal is more useful, but it is time
consuming.
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45. STEP 5: COMPARING WITH KNOWN
INDICES
Comparisons are made with known
indices to arrive at clues to the disease’s
etiology
45

46. STEP 6: FORMULATION OF A
HYPOTHESIS
A hypothesis is a supposition, arrived at from
observation or reflection
o An epidemiologic hypothesis should specify-
 The population- characteristics of the people to
whom the hypothesis applies
 Specific cause
 Expected outcome-the disease
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47.  Dose response relationship-The amount of
the cause needed to lead to the stated
incidence of the effect.
 Time response relationship-Time period
between exposure to the cause and
observation of the effect.
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48. USES OF DESCRIPTIVE EPIDEMIOLOGY
 It provides data regarding the magnitude of
the disease load and types of disease
problems in terms of morbidity and mortality
rates and ratios.
 It provides clue to disease etiology and help in
the formulation of an etiological hypothesis.
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49.  It provides background data for planning,
organizing and evaluating preventive
service.
 Contribute to research by describing
variations in disease occurrence by time, place
and person.
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50. THANK YOU
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