Descriptive Epidemiology (including Measurement in epidemiology)

Dr. Animesh Gupta
MBBS,MD,FDM,FAGE
Associate Professor, Dept. of Community Medicine, NMCH, Sasaram (Bihar)
4/25/2020Epidemiology - Dr. Animesh Gupta
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Measurement of
▪ Mortality
▪ Morbidity
▪ Disability
▪ Presence or absence or distribution of characteristic or
attributes of disease
▪ Health care or health utilization
▪ Demographic variables
4/25/2020Epidemiology - Dr. Animesh Gupta 2

▪ Incidence – the no. of NEW cases
occurring in a defined population
during a specified period of time".
▪ Prevalence – all current cases (old
and new) existing at a given point in
time, or over a period of time in a
given population.
▪ Prevalence = Incidence * Duration of
illness

▪Rate
▪ It measures the occurrence of particular event in a population
during given time period.
▪ A rate comprises the following elements - numerator, denominator,
time specification and multiplier.
▪ Example:
Death rate = No. of death in a year/ Mid year population * 1000

▪Ratio
▪ It expresses a relation in size between two random quantities.
▪ The numerator is not a component of the denominator.
▪Proportion
▪ It is a ratio which indicates the relation in magnitude of a part of the
whole.
▪ The numerator is always included in the denominator.
▪ It is usually expressed as a percentage.

▪Descriptive
▪Analytic
▪Experimental
▪Study of the occurrence and
distribution of disease
▪Further studies to determine the
validity of a hypothesis concerning
the occurrence of disease.
▪Deliberate manipulation of the cause
is predictably followed by an
alteration in the effect not due to
chance

OBSERVATIONAL STUDIES
DESCRIPTIVE STUDY ANALYTICAL STUDIES
✓ TIME
✓ PLACE
✓ PERSON
➢ ECOLOGICAL STUDY
➢ CROSS SECTIONAL STUDY
➢ CASE-CONTROL STUDY
➢ COHORT STUDY
EXPEREMENTAL STUDIES
➢ RANDOMIZED CONTROLLED TRIAL (RCT)
➢FIELD TRIAL
➢COMMUNITY TRIAL 4/25/2020 8Epidemiology - Dr. Animesh Gupta

▪1st phase of an epidemiological investigation
▪Descriptive analysis is limited to a description of the
occurrence of a disease in a population & identifying the
characteristics associated with it
▪Relatively inexpensive and less time-consuming than
analytic studies.

▪Patterns of disease occurrence, in terms of,
▪When is the disease occurring – time distribution
▪Where is it occurring – place distribution
▪Who is getting a disease – person distribution

▪ Defining the population to be studied
▪ Defining the disease under study
▪ Describing the disease by
✓ Time
✓ Place
✓ Person
▪ Measurement of disease
▪ Comparing with known indices
▪ Formulation of etiological hypothesis

▪Define the population in
relation to:
1. Number
2. Age
3. Gender
4. Occupation
5. Cultural and other
characteristics
▪The defined population
can be:
1. Whole population
2. Sample
3. Specially selected
groups

▪Large enough
▪Stable (no migration)
▪Clear on who belongs to the population
▪Community participation

▪The epidemiologist defines the disease which can be
measured and identified in the defined population with a
degree of accuracy (operational definition).
▪This is different from the clinician’s definition of a disease.
▪Example - tonsillitis might be defined clinically as an
inflammation of the tonsils caused by infection, usually with
streptococcus pyogenes.
▪OD - presence of enlarged, red tonsils with white exudate,
which on throat swab culture grow predominantly S.
pyogenes.

TIME PLACE PERSON
Climatic zones Age Birth order
Year, season
Month, week
Country,
region
Gender
Marital state
Family size
Height, weight
Day, hour
Duration
Urban/rural
Towns, cities,
institutions
Occupation
Social status
Education
BP.
Cholesterol,
habits
Objective of descriptive epidemiology?
This involves systemic collection and analysis of data.

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
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▪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”

▪ A graph of time distribution ofepidemic cases is called epidemic curve.

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▪ Common source epidemics-
a) Single/Point exposure
b)Continuous/Multiple exposure
▪ Propagated epidemics
a) person to person
b)arthropod vector
c) animal reservoir
▪ Slow(modern) epidemics

▪ 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) 14

▪ 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.

▪The epidemic shows gradual rise and tails off over a much
longer period of time
▪It is more likely to occur where there is
▪large number of susceptible are aggregated
▪regular supply of new susceptible individuals (Births,
Immigrants)
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COURSE OF A TYPICAL PROPAGATED EPIDEMIC

▪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.
▪Example - Sunstroke, hay fever
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▪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 are more frequent on
weekends.

▪It refers to changes in the occurrence of disease over a
long period of time.
▪Eg: Coronary disease, diabetes showing consistent
upward trend,
▪decline in TB, polio in developed countries during the
past 50 yrs.

2) PLACE DISTRIBUTION
Place
variations
International
Variation
National variation
Rural-Urban
variation
Local distribution 25

InternationalVariation
▪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.
NationalVariation
▪There are variations in
disease occurrence
within countries.
▪Example:
▪Endemic goitre,
▪Fluorosis
▪Malaria
▪Nutritional deficiencies

▪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

▪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

▪Way of distinguishing genetic and environmental factors.
▪Study of genetically similar groups but living under
different environmental conditions.
▪ Eg: Twins
▪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

▪Age
▪Gender
▪Marital status
▪Occupation
▪Ethnicity
▪Social factors like customs, Social class
▪Habits
▪Migration

▪Cross sectional study – Prevalence
▪It is based on a single examination of a cross section of
population at one point in time.
▪More useful for chronic diseases
▪Longitudinal Study – Incidence
▪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.
▪Mortality, Morbidity & Disability

▪Making comparison
▪Asking question
▪It helps to
▪Arrive at clues to the disease’s etiology
▪Identify groups at increased risk

▪It is a supposition, arrived at from observation or
reflection
▪It should specify following:
▪ Population
▪ Specific cause being considered
▪ Expected outcome – the disease
▪ Dose-response relationship
▪ Time-response relationship

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

▪Epidemiological methods (Classification)
▪Descriptive epidemiology
▪Defining the population
▪Defining the disease under study
▪Describing the disease
▪Measurement of disease
▪Comparing with known indices
▪Formulation of etiological hypothesis
▪Uses
Steps

Descriptive Epidemiology (including Measurement in epidemiology)

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