3. Introduction
• The best study of mankind is man.
• Descriptive studies are usually the first phase of an
epidemiological investigation.
• These studies are concerned with observing the distribution
of disease or health related characteristics in human
populations and identifying the characteristics in which the
disease in question seems to be associated.
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5. Cross Sectional Survey
• Takes place at a single point in time
• Measure disease and exposure simultaneously
in a well-defined population
• Often used to look at the prevalence of
something in a given population
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6. Longitudinal Survey
• Observations are repeated in the same population or
different populations over a period of time through
follow up examinations.
Advantages
1) To study the natural history of disease and its future
outcome
2) For identifying risk factors
3) Rate of occurrence of new cases of disease
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7. • Limitations
1) Difficult to organize and more time
consuming
2) Attrition is more common as time goes by
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8. 8
Descriptive epidemiology
• When is the disease occurring?
Time distribution
• Where is it occurring ?
Place distribution
• Who is getting the disease?
Person distribution
“I keep six honest serving Men. They taught
me all I Know. Their names are – what, why,
when, how, where and who.”
Rudyard Kipling 1903
9. Procedures in Descriptive Studies
1. Defining the population to be studied
2. Defining the disease under study
3. Describing the disease under study
4. Measurement of disease
5. Comparing with known indices
6. Formulation of an etiological hypothesis
10. 1.Defining the population to be studied
• Investigations of populations and not an individual
• Define “population base”.
• Defined population
-Whole population: (geographic area)
- Representative sample
• Large enough and stable
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11. • The concept of “defined population”(or population at risk) is
crucial in epidemiological studies. It provides the denominator
for calculating rates, which are essential to measure the
frequency of disease and study its distribution and
determinants.
• Epidemiologists have been labeled as men in search of a
denominator
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12. 2.Defining the disease under study
• Epidemiologist needs a precise and valid definition
• Operational definition -
a definition by which the disease or condition can be
identified and measured in the defined population with a
degree of accuracy.
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14. Characteristics frequently examined in
descriptive studies
Time Place Person
Year, Season Climatic zones Age Birth order
Month, week
Country,
region Sex Family size
Day, hour of
onset
Urban/Rural
Local
community Marital status Height/Weight
Duration
Towns cities
Institutions
Occupation
Social status
Education
Blood Pressure
Blood
Cholesterol
Personal
habits
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17. I. Short term fluctuations
• Best short term fluctuation is epidemic.
• According to the modern concepts an epidemic is defined
as “ the occurence 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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18. Epidemic curve
It is a histogram that displays the course of an outbreak or
epidemic by plotting the number of cases according to time of
onset.
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19. It suggests:
• A time relationship with exposure to a suspected
source.
• A cyclic or seasonal pattern suggestive of a particular
infection
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20. A) Common Source Epidemic
• Common source ,single exposure epidemics
Known as ‘point source’ epidemic.
Exposure to the agent is brief and essentially
simultaneous, the resultant cases all develop within one
incubation period of the disease.
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21. Common source epidemics are frequently, but not always, due to
exposure to an infectious agent.
E.g. : Food poisoning, Bhopal gas tragedy
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22. Common source, continuous or multiple exposure
• Exposure from the source may be prolonged – continuous,
repeated or intermittent- not necessarily at the same time or
place.
• A variation may be that an epidemic may be initiated from
a common source and then continue as a propagated
epidemic.
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23. E.g. : Water from contaminated well, Nationally distributed
brand of vaccine or food could result in similar out breaks.
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24. B) Propagated epidemic
• Most often of infectious origin and results from person to
person transmission of the agent.
• Epidemic shows gradual rise and tails of over a much
longer period of time.
• E.g.: Epidemics of Hepatitis A
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25. • Transmission continues until the number of susceptible is
depleted or susceptible individuals are no longer exposed to
infected persons or intermediary vectors.
• Speed of spread depends on
- Herd immunity
- Opportunities for contact
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26. II. Periodic Fluctuations
a) Seasonal trend –
Seasonal variation is a well known characteristic of many
communicable diseases like measles, varicella, sunstroke ,
upper respiratory infections
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27. b) Cyclic trend –
Some diseases occur in cycles spread over short periods of
time which may be days, weeks, months or years.
E.g.- Measles appeared in cycles with major peaks every 2-3
years and rubella every 6-9 years.
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28. III. Long term or secular trends
It implies changes in the occurrence of disease over a long
period of time( i.e., a progressive increase or decrease)
generally several years or decades.
E.G : Coronary heart disease, lung cancer, diabetes have shown
a consistent upward trend and diseases like Tuberculosis ,
typhoid fever, diphtheria & polio have shown a downward
trend.
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29. • Interpretation of time trends :
- Helps us to know which diseases are increasing and which
are the emerging health problem and measures to control the
old problems.
- By studying the time trends, the epidemiologist seeks to
provide guidelines to the health administrator in matters of
prevention or control of diseases.
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31. • International Variations :
E.g. : Japan – High rates of death from
stomach cancer but unusual in US
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32. • National variations :
E.g. : Endemic goiter, Fluorosis etc.
• Rural – Urban variations :
E.g. : Rural areas - Skin and zoonotic infections and
nutritional deficiencies.
Urban areas – Chronic Bronchitis, Accidents, C-V
diseases
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35. 4.Measurement of disease
• - Measurement of mortality and Morbidity
- Morbidity has two aspects – Incidence and prevalence.
Incidence can be obtained from Longitudinal studies.
Prevalence can be obtained from Cross sectional studies.
• Cross sectional studies are more useful for the study of chronic
diseases than short lived.
• Longitudinal studies are difficult to organize and more time-
consuming.
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36. 5.Comparing with known Indices
• By making comparisons between different populations and
sub groups of the same population, it is possible to arrive at
clues to disease etiology.
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37. 6.Formulation of etiological hypothesis
• Hypothesis is a supposition, arrived at from
observation or reflection.
• An epidemiological Hypothesis should specify the
following:
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38. a) The population – The characteristics of the persons to
whom the hypothesis applies
b) The specific cause being considered
c) The expected outcome – the disease
d) The dose response – relationship
e) The time response – relationship
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39. An improved formulation :
“ The smoking of 30-40 cigarettes per day causes lung cancer in
10% of smokers after 20 years of exposure.”
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40. Uses of Descriptive epidemiology
• To provide data regarding the magnitude of the disease load
and types of disease problems in the community in terms of
morbidity and mortality rates and ratios.
• Provides clues to disease etiology and helps in the
formulation of etiological hypothesis.
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41. • Provides background data for planning, organizing and
evaluating preventive and curative services
• They contribute to research by describing the variations in
disease occurrence by time, place and person
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42. Conclusion
For epidemiologists, the identification of descriptive
characteristics frequently constitutes an important
step in search for determinants or risk factors that
can be altered or eliminated to reduce or prevent
disease.
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43. References
• Park K. Textbook of Preventive and Social Medicine. 23rd ed.
Bhanot Publishers. Pg no.62-73
• Peter S. Essentials of Preventive and Community Dentistry. 4th
ed. Arya Medical Publishers Pg no.92-104.
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