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Descriptive Epidemiology

Presented by : Dr. Vini Mehta MDS 1st Year

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Contents

• Introduction• Classification of Epidemiological studies• Descriptive Epidemiology• Uses of Descriptive Epidemiology• Conclusion • References

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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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EPIDEMIOLOGICAL STUDIES

OBSERVATIONAL STUDIES

DESCRIPTIVE

STUDIES

ANALYTICALSTUDIES

ECOLOGICAL STUDIES

CROSS-SECTIONALSTUDIES

CASE

CONTRO

L

STUDIES

COHORT STUDIES

EXPERIMENTAL STUDIES

RANDOMIZED

CONTROLLED TRIALS

FIELD TRIALS

COMMUNITY TRIALS

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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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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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• Limitations

1) Difficult to organize and more time

consuming

2) Attrition is more common as time goes by

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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

Procedures in Descriptive Studies

1. Defining the population to be studied2. 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

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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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• 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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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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3.Describing the disease

• Describing the disease by

- Time

- Place

- Person

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Characteristics frequently examined in descriptive studies

Time Place Person

Year, Season Climatic zones Age Birth order

Month, weekCountry, region Sex Family size

Day, hour of onset

Urban/Rural Local

community Marital status Height/Weight

DurationTowns cities Institutions

Occupation Social status

Education

Blood PressureBlood

CholesterolPersonal

habits

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Time distribution

• Short term fluctuations

• Periodic fluctuations

• Long term or secular trends

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Types Short Term

Common

sourceSi

ngle exposure

continuous

Propagated

Person-person

Arthropod Slow Animal

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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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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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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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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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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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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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E.g. : Water from contaminated well, Nationally distributed

brand of vaccine or food could result in similar out breaks.

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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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• 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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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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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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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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• 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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Place Distribution

International variations

National variations

Rural / Urban variations

Local variations

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• International Variations :

E.g. : Japan – High rates of death from

stomach cancer but unusual in US

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• 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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Person Distribution

Some characteristics of a person have been identified as risk factor for a disease. These variables include :

Age

Sex

Ethinicity

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Marital status

Occupation

Social class

Behaviour

Stress

Migration

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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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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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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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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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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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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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• 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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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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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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• http://www.thelancet.com/journals/lancet/article/PIIS0140-6736%2802%2907373-7/fulltext

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