Introduction to Descriptive Epidemiology.pptx

DESCRIPTIVE EPIDEMIOLOGY
Professor Dr. AB Rajar, MBBS, Dip-Diab, MPH, Ph.D.CPHE
Director of Research and Innovative Center
[IBN-E-SINA UNIVERSITY]
Professor Dr AB Rajar/drabrajar@gmail.com 1

How we view the world
• Pessimist:
• The glass is half empty.
• Optimist:
• The glass is half full.
• Epidemiologist:
• As compared to what?

Learning Objectives
• Introduction to descriptive epidemiology
• Definition, basics terminology & Key concept.
• Classification of study designs
• Study protocol of descriptive epidemiology [steps]
• Uses of descriptive epidemiology
• Conclusion

Epidemiology
• Epidemiology (Epi, meaning on or upon, Demos, meaning
people, Logos, meaning the study)
• Epidemiology is the study of the
1. Distribution &
2. Determinants of health-related states or events in
specified populations,
3. The application of this study to the control of health
problems
John M. Last (1988

Key Concept of Epidemiology
Frequency of Disease
Distribution of
Disease
Application of control of
the disease
Determinant of
disease

Description of terms
• Distribution :
– Frequency of health events in a
population
– Pattern of health events in a population
• Determinants:
– Are the causes and other factors that
influence the occurrence of disease and
other health-related events
– illness happens only when the right
accumulation of risk factors or
determinants exists in an individual

Frequency
• Frequency :
• number of cases of disease (meningitis, diabetes) or any
condition in a population
• & relationship of that number to the size of the population

Pattern
Time
Person
Place
Time
Annual, seasonal,
weekly, daily, hourly,
weekday versus
weekend,
Personal
characteristics which
may be related to risk
of illness, injury, or
disability
Place
Geographic variation,
National international
urban/rural differences

Determinants
• Demographic characteristics [Age, Gender,
Urban, Rural]
• Genetic or immunologic makeup,
• Behaviors, environmental exposures,
• Other so-called potential risk factors.
• Ideally, the findings provide sufficient
evidence to direct prompt and effective
public health control and prevention
measures.

Descriptive V Analytical Epidemiology
Descriptive
• To search for frequency and
pattern (time, place, and person),
epidemiologists use descriptive
epidemiology or epidemiologic
studies to provide the “When”
“Where” and “Whom” of such
events
Analytical
• To search for these determinants,
epidemiologists use analytic
epidemiology or epidemiologic
studies to provide the “Why” and
“How” of such events.

Descriptive V Analytical Epidemiology
Professor Dr AB Rajar/drabrajar@gmail.com
11
Epidemio
logist
Where
Who
How
Why
When
Descriptive Study
Analytical Study

Types of Epidemiology
Epidemiology
Observational
Descriptive Analytical
Experimental
RCT
Field
Community
trial

Types of Epidemiology
• OBSERVATIONAL STUDIES (Not Interventional)
• Descriptive
1. Case report
2. Case Series
3. Cross-sectional descriptive
• Analytical
1. Cross-Sectional analytic
2. Case Control
3. Cohort
• EXPERIMENTAL STUDIES (Interventional)
• RCT
• Field & Community trial

• “ 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

• First phase of an epidemiologic
investigation
Concerned with:

STEPS IN DESCRIPTIVE STUDIES
1. Defining the population
2. Defining the disease under study
3. Describing the disease
I. Time
II. Place
III.Person
4. Measurement of disease
5. Comparing with known indices
6. Formulation of etiological hypothesis

Defining the population
• "Defined population" can be
• Whole population, or
• Representative sample
• Specifically selected groups such as age, sex,
occupational groups, school children, pregnant
mothers, etc.
• The concept of a defined population is critical
because it provides the denominator for calculating
rates.

Describing the study population
Total Number
Age
composition
Sex
composition
Occupations
Socioeconomic
status.
Social customs,
Habits, Norms,
Values.
Specific
life-styles.
Knowledge of health
facilities available
and their utilization.
Literacy
profile

Defined population should be
• Its representativeness to the parent reference
population
• Stable (no migration)
• Clear on who belongs to the population
• Community participation
• Its optimum size:
• Then only the results of the hypothesis tested on any study
population can be generalized to the reference population.
• This step is very important as it forms the bases of all calculations

I. Time
II. Place
III.Person

•Need for an operational definition
•Why is it important??
•Epidemiologist needs a definition that is precise and valid to obtain an
accurate estimation about the disease.
• What is an operational definition.
• A definition by which the disease or condition can be identified and
measured in the defined population with a degree of accuracy
• This is required so as to enable observer to identify those who have the
disease from those who do not have.

Disease Definition
• Tonsillitis is defined as an
inflammation of tonsils, caused by
infection, usually with streptococcus
pyogenes.
Clinical
definition
• Tonsillitis would include the presence of
enlarged, red tonsils with white
exudates which on throat swab culture
grow predominantly S.pyogenes.
Operational
definition

Disease Definition
Definition
• Gingivitis
• Inflammation of the
gingiva.
• Dental caries:
• Infectious microbial disease
affecting the calcified
tissues of teeth.
Operational definition
• Gingivitis
• Gingival bleeding in one or more
sites after gently probing the
gingival sulcus.
• Dental caries:
• The lesion is clinically visible and
obvious. Explorer tip can penetrate
deep into soft yielding material.
• There is discoloration or loss of
translucency. Explorer tip resists
removal after moderate to firm
pressure

I. Time
II. Place
III.Person

3. DESCRIBING THE DISEASE
• Objective of descriptive epidemiology?
• This involves systemic collection and analysis of data.
TIME PERSON PLACE
Year, Season Climate zones, Age Birth order
Month, week Country, region Gender Family size
Day, hour Urban/rural Marital status. Height, weight
Duration
Towns, cities,
institutions.
Occupation social
status. Education
BP, Cholesterol,
habits

TIME DISTRIBUTION
Time distribution in descriptive epidemiology

TIME DISTRIBUTION
Short time
fluctuations
• Hours, days, week
[Epidemics]
Periodic
Fluctuation
• [Months]=cyclical
trends/seasonal
variations
Long term or
secular trends
• Usually
10 years

SHORT TIME FLUCTUATIONS.
Hours, Days, Weeks Epidemics

Short time fluctuations.
• Epidemic:
• Def.- " The occurrence of cases of an illness or other
health related events in a region or a community clearly
in excess of the normal expectancy.
Epidemic curve — A graph of the time distribution of
epidemic cases.
It shows time relationship with exposure to a
suspected source.

Epidemic curve [Point source]

Short time fluctuations.
A-Common source
epidemics
• Single exposure or
‘point source
‘epidemics
• Continuous or multiple
exposure epidemics.
B-Propagated
epidemics
• Person to person
• Arthropod vector
• Animal reservoir
C-Slow (modern)
epidemics

A-COMMON SOURCE EPIDEMICS
• 1-Single exposure or ‘point source ‘epidemics:
• All cases develop almost simultaneously following single
exposure. e.g. food poisoning, Bhopal gas tragedy.
• Features:
• There is the sudden rise and sudden fall
• There are no secondary curves.
• Large number of cases occur with a narrow interval of time
• All cases have the same incubation period
• Exposure is almost simultaneous and brief.

Common vehicle, point source Epidemic Curve

A-COMMON SOURCE EPIDEMICS
• 2-Continuous/Multiple Exposure Epidemics:
• It occurs when the exposure from the same source is
prolonged and the epidemic continuous over more than one
incubation period.
• The epidemic reaches a sharp peak, but tails off gradually
over a longer period of time.
• E.g.: A well of contaminated water or nationally distributed
vaccine (polio vaccine) or food; waterborne cholera.

Common Source Continuous Curve

B-PROPAGATED EPIDEMICS
• The propagated epidemics are most often of infectious origin and
usually result from person-to-person, arthropod, or animal
transmission of the infectious agent.
• e.g. Hepatitis A, poliomyelitis, Measles.
• Characteristics:
 A gradual rise & tails off over a long period of time.
 Transmission continues till susceptible individuals are
exposed to infected persons.
 Speed of spread depends upon Herd immunity.

Propagated Epidemic Curve

Propagated Epidemic

C-SLOW (MODERN) EPIDEMIC
• These epidemics occur over a very long period of time.
• Like:
I. Hypertension
II. Coronary heart diseases
III. Cancers

Time distribution
Short time
fluctuations
[Epidemics]
Periodic
Fluctuation
trends/seasonal
variations
Long term or
secular trends
• Usually
10 years

Periodic Fluctuation
[Months]=cyclical trends/seasonal variations

Periodic Fluctuations

• SEASONAL TRENDS:
– Communicable diseases RTI, Measles, Cholera, and others
– communicable Hay fever, Sunstroke, Snakebite

• CYCLICAL TRENDS:
– Some diseases appear in cycles which may be spread over short periods of time like
days,weeks, months or years like:
– Influenza pandemic (7-10 years), Auto Mobil Accident (week ends)

Time distribution
Short time
fluctuations
[Epidemics]
Periodic
Fluctuation
trends/seasonal
variations
Long term or
secular trends
• Usually
10 years

LONG-TERM OR SECULAR TRENDS
• Changes occur over a long period of time. (usually >10
years).
• It could be increasing or decreasing, real or apparent,
communicable or non-communicable diseases.
• E.g.
1. Downward trend for Plague, Typhoid fever, Diphtheria, TB &
cholera.
2. Upward trend for DM, CHD, and Lung cancer.

LONG-TERM OR SECULAR TRENDS.

Interpretation of time trends:
• Helps us to know which diseases are increasing and which are
emerging health problems and measures to control the old
problems.
• By studying the time trends, epidemiologists seek to provide
guidelines to the health administrator in the matter of prevention
or control of the disease.

PLACE DISTRIBUTION
International, National, Rural/Urban, Local

Place distribution
• Geographical comparisons:
• Differences in disease patterns between countries and within
countries.
• Importance of genes versus environment.
• Changes with migration and role of diet.
• Culture and standard of living and external environment vary in
different parts of the world.
Place distribution in descriptive epidemiology

Place distributions
International
Variations
National Variations
Rural/Urban
Variation
Local Variations
Distribution

Place distributions
International
• Malaria,
Leprosy in hot
& humid
climate [Africa,
Asia]
• Ca stomach -
Japan
• Ca oral cavity &
Ca cervix –
India n Pak
National
• Endemic goiter
[ hilly areas]
• Malaria, filaria,
GIT disorders.
Local
• Studied with
spot map in a
given area
• Endemic
fluorosis in
Achhro thar
[desert]
Urban/Rural
• Tetanus, OP
poisoning, and
zoonotic are in
rural.
• RTA, drug
abuse, DM,
cancer, and
mental stress
are more
common in
urban.

International Variations

Spot map
• It is a graphical presentation of the place distribution of
the disease of occurrence.
• “clustering” of cases suggests a common source of
infection & mode of spread.
• E.g. investigation of the cholera epidemic by john snow of
England with help of a spot map.
• Factors influencing geographical variations are culture,
standards of living, external environment, and genetic
factors.

Geographical Spot maps

PERSON DISTRIBUTION
Age, Gender, Marital Status, Ethnicity, Occupation, Social class,
Behavior, Stress, Migration.

Person Distribution
• AGE:
• If equal susceptibility in all ages – no previous immunity
• Progressive increase with age – persistent exposure to the
causative agent.
• Measles in childhood, Cancer in the middle, and
Atherosclerosis in advanced age.
• GENDER: Male/Female
• Biological and cultural differences.
• ETHNICITY:
• Differences in disease occurrence in different population
subgroups.
Person distribution in descriptive epidemiology

Person Distribution
• MARTIAL STATUS :
• Mortality rate decreases among married women as compare to
unmarried
• Reasons
• Feel more secure & protected
• Spend Sober life
• Increase chance of acquiring STIs and ca of the cervix
• STRESS:
• Susceptibility to diseases
• Exacerbation of symptoms
• Compliance with medical regimen.

Person Distribution
• OCCUPATION:
• Occupational disorders like Byssinosis, Asbestosis, and Bagassosis.
• SOCIAL CLASS:
• Higher social class- Diabetes, coronary heart diseases,
• Lower social class: Nutritional disorders.
• BEHAVIOR:
• Cigarette smoking, sedentary lifestyle, drug abuse, overeating.
• MIGRATION:
• Comparison of migrants with the population of the host country
• Malaria from rural to urban areas.

Age, Social Status, Migration, Marital Status

Person Distribution Age Modality
Distribution of cervical cancer frequency showing the bimodal curve

Person Distribution Age Modality

I. Time
II. Place
III.Person

4.Measurement of disease
• Knowing “DISEASE LOAD” is important.
• Measured based on:
• Mortality
• Morbidity
• Disability
• Mortality – straightforward
• Morbidity – incidence, and prevalence
• Incidence – longitudinal studies
• Prevalence – cross-sectional studies.

• INCIDENCE
• Definition: Incidence is defined as the occurrence of
new cases of disease that develop in a population over
a specified time period.
• Incidence =
𝑇𝑜𝑡𝑎𝑙 𝑛𝑜 𝑜𝑓 𝑝𝑒𝑜𝑝𝑙𝑒 𝑎𝑓𝑓𝑒𝑐𝑡𝑒𝑑 𝑤𝑖𝑡ℎ 𝑐𝑎𝑟𝑟𝑖𝑒𝑠 𝑜𝑣𝑒𝑟 𝑎 𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑒𝑑 𝑡𝑖𝑚𝑒 𝑝𝑒𝑟𝑖𝑜𝑑
𝑇𝑜𝑡𝑎𝑙 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛
ᕁ1000

• PREVALENCE
• Definition: All current cases [old & new] existing at a
given point in time or over a period of time in a given
population.
• Types:
• Point Prevalence:
• Proportion of the population that is diseased at a single point
in time. It is a single snapshot of the population.
• Period Prevalence:
• Proportion of the population that is diseased during a
specified period of time.

I. Time
II. Place
III.Person

5-COMPARING WITH KNOWN INDICES
A. Making comparison
B. Ask questions between
• Different populations or
• Subgroups of the same population.
1. It is possible to arrive at clues to disease etiology
2. Identify groups that are at ‘high risk’ for the disease.

1. ASKING QUESTIONS
Related to Health Events
• What is the event? (Problem)
• What is magnitude?
• Where did happen?
• When did happen?
• Who is affected?
• Why did it happen?
Related to Health Action
• What can be done to reduce the
problem?
• How can be prevented in the
future?
• What action should be taken by
the community?
• What resources are required?
• How are activities to be
organized?
• What difficulties may arise?
Epidemiology is "a means of learning by asking questions and getting
answers that lead to further questions."
70

2. MAKING COMPARISONS
• To find out the differences in the AGENT, HOST, and
ENVIRONMENT conditions between the two groups.
• Weighs. balances and contrasts give clues to
ETIOLOGICAL HYPOTHESIS.

• Infectivity
• Pathogenicity
• Virulence
•Immunogenicityy
• Antigenic stability
• Survival
• Weather
• Housing
• Geography
• Occupationnel setting
• Air quality
• Food
Environment
• Age
• Sex
• Genotype
• Behaviour
• Nutritional status
• Health status
Host
Agent
Factors Influencing Disease Transmission

I. Time
II. Place
III.Person

6-FORMULATION OF ETIOLOGICAL HYPOTHESIS
• Hypothesis is supposition, arrived at from observation or
reflection of steps 1 to step 5
• Hypothesis may be accepted or rejected by analytical study
• Should specify the following
1. Population
2. Specific cause
3. Expected outcome
4. Dose-response relationship
5. Time response relationship

6-FORMULATION OF ETIOLOGICAL HYPOTHESIS
An epidemiological hypothesis should specify
1. Population (characteristics of persons)
2. The specific cause
3. The expected outcome – disease
4. The dose-response relationship
5. Time response relationship
• E.g. The smoking of 30-40 cigarettes per day causes lung
cancer in 10% of smokers after 20 years of exposure.

USES OF DESCRIPTIVE STUDY
• Contributes to research by describing variations in disease
occurrence by time, place, and person.
• Clues to disease epidemiology – aetiological hypothesis.
• Data regarding the magnitude of disease load and types of
disease problems in the community in terms of morbidity
and mortality.
• Background data for planning, organizing, and evaluating
preventive and curative services.

Conclusion
• For Epidemiologists;
• The identification of descriptive characteristics frequently
constitutes an important step in the search for determinants
or risk factors that can be altered or eliminated to reduce or
prevent disease.

Thank You For Your Attention End

Introduction to Descriptive Epidemiology.pptx

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