Basic measurement in epidemiology
Incidence & Prevalence
Tools of measurement in epidemiology
Epidemiological methods
Descriptive epidemiology.
Distribution of disease in term of Time, Place and Person
2. Measurement of
â–Ş Mortality
â–Ş Morbidity
â–Ş Disability
â–Ş Presence or absence or distribution of characteristic or
attributes of disease
â–Ş Health care or health utilization
â–Ş Demographic variables
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▪ 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.
â–Ş Prevalence = Incidence * Duration of
illness
5. â–ŞRate
â–Ş It measures the occurrence of particular event in a population
during given time period.
â–Ş A rate comprises the following elements - numerator, denominator,
time specification and multiplier.
â–Ş Example:
Death rate = No. of death in a year/ Mid year population * 1000
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6. â–ŞRatio
â–Ş It expresses a relation in size between two random quantities.
â–Ş The numerator is not a component of the denominator.
â–ŞProportion
â–Ş It is a ratio which indicates the relation in magnitude of a part of the
whole.
â–Ş The numerator is always included in the denominator.
â–Ş It is usually expressed as a percentage.
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7. â–ŞDescriptive
â–ŞAnalytic
â–ŞExperimental
â–ŞStudy of the occurrence and
distribution of disease
â–ŞFurther studies to determine the
validity of a hypothesis concerning
the occurrence of disease.
â–ŞDeliberate manipulation of the cause
is predictably followed by an
alteration in the effect not due to
chance
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8. OBSERVATIONAL STUDIES
DESCRIPTIVE STUDY ANALYTICAL STUDIES
âś“ TIME
âś“ PLACE
âś“ PERSON
➢ ECOLOGICAL STUDY
➢ CROSS SECTIONAL STUDY
➢ CASE-CONTROL STUDY
➢ COHORT STUDY
EXPEREMENTAL STUDIES
➢ RANDOMIZED CONTROLLED TRIAL (RCT)
➢FIELD TRIAL
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9. â–Ş1st phase of an epidemiological investigation
â–ŞDescriptive analysis is limited to a description of the
occurrence of a disease in a population & identifying the
characteristics associated with it
â–ŞRelatively inexpensive and less time-consuming than
analytic studies.
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10. â–ŞPatterns 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
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11. â–Ş Defining the population to be studied
â–Ş Defining the disease under study
â–Ş Describing the disease by
âś“ Time
âś“ Place
âś“ Person
â–Ş Measurement of disease
â–Ş Comparing with known indices
â–Ş Formulation of etiological hypothesis
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12. â–ŞDefine the population in
relation to:
1. Number
2. Age
3. Gender
4. Occupation
5. Cultural and other
characteristics
â–ŞThe defined population
can be:
1. Whole population
2. Sample
3. Specially selected
groups
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13. â–ŞLarge enough
â–ŞStable (no migration)
â–ŞClear on who belongs to the population
â–ŞCommunity participation
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14. â–ŞThe 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.
â–ŞExample - tonsillitis might be defined clinically as an
inflammation of the tonsils caused by infection, usually with
streptococcus pyogenes.
â–ŞOD - presence of enlarged, red tonsils with white exudate,
which on throat swab culture grow predominantly S.
pyogenes.
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15. 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.
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17. â–ŞAn 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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18. â–Ş A graph of time distribution ofepidemic cases is called epidemic curve.
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â–Ş Common source epidemics-
a) Single/Point exposure
b)Continuous/Multiple exposure
â–Ş Propagated epidemics
a) person to person
b)arthropod vector
c) animal reservoir
â–Ş Slow(modern) epidemics
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20. â–Ş 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
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22. â–Ş 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.
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24. â–ŞThe epidemic shows gradual rise and tails off over a much
longer period of time
â–ŞIt is more likely to occur where there is
â–Şlarge number of susceptible are aggregated
â–Şregular supply of new susceptible individuals (Births,
Immigrants)
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26. COURSE OF A TYPICAL PROPAGATED EPIDEMIC
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27. â–ŞSeasonal trend-
â–ŞSeasonal variation is characteristic of many
communicable diseases.
â–ŞEg: Measles, upper respiratory tract infections(seasonal
rise during winter), Malaria, etc.
â–ŞNon-infectious diseases and conditions may sometimes
exhibit seasonal variation.
â–ŞExample - Sunstroke, hay fever
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29. â–ŞSome diseases occur in cycles spread over short periods
of time (days, weeks, months or years) .
â–ŞEg: Influenza pandemics are known to occur at intervals
of 7-10yrs due to antigenic variations.
â–ŞNon-infectious conditions may also occur in this trend.
â–ŞEg: Automobile accidents are more frequent on
weekends.
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30. â–ŞIt refers to changes in the occurrence of disease over a
long period of time.
â–ŞEg: Coronary disease, diabetes showing consistent
upward trend,
â–Şdecline in TB, polio in developed countries during the
past 50 yrs.
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32. InternationalVariation
â–ŞDescriptive studies have
shown that the pattern
of a disease is not the
same everywhere
â–ŞEg: Cancer of the
stomach is very
common in Japan, but
unusual in the US.
NationalVariation
â–ŞThere are variations in
disease occurrence
within countries.
â–ŞExample:
â–ŞEndemic goitre,
â–ŞFluorosis
â–ŞMalaria
â–ŞNutritional deficiencies
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33. â–ŞDue to differences in population density, levels of
sanitation, deficiencies of medical care, education and
environment factors, there exists a rural-urban variation
â–ŞChronic bronchitis, cardiovascular diseases, accidents are
more frequent in urban than rural areas.
â–ŞSkin and zoonotic diseases and soil transmitted helminths
may be more frequent in rural than urban areas
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34. â–ŞThese variations can be
studied with the help of “spot
or shaded” maps.
â–ŞIf the map showed clustering,
it may suggest a common
source of infection.
â–ŞEg: Study of Cholera epidemic
by John Snow in 1854
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35. â–ŞWay of distinguishing genetic and environmental factors.
â–ŞStudy of genetically similar groups but living under
different environmental conditions.
â–Ş Eg: Twins
â–ŞStudy of genetically different groups living in a
similar environment.
â–Ş Eg: Men of Japanese origin living in USA have higher rate of
coronary heart disease than the Japanese in Japan
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37. ▪Cross sectional study – Prevalence
â–ŞIt is based on a single examination of a cross section of
population at one point in time.
â–ŞMore useful for chronic diseases
▪Longitudinal Study – Incidence
â–ŞThe observations are repeated in the same population
over a prolonged period of time by means of follow up
examination.
â–ŞLongitudinal is more useful, but it is time consuming.
â–ŞMortality, Morbidity & Disability
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38. â–ŞMaking comparison
â–ŞAsking question
â–ŞIt helps to
▪Arrive at clues to the disease’s etiology
â–ŞIdentify groups at increased risk
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39. â–ŞIt is a supposition, arrived at from observation or
reflection
â–ŞIt should specify following:
â–Ş Population
â–Ş Specific cause being considered
▪ Expected outcome – the disease
â–Ş Dose-response relationship
â–Ş Time-response relationship
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40. â–ŞContributes to research by describing variations in
disease occurrence by time, place and person
▪Clues to disease epidemiology – aetiological
hypothesis
â–ŞData regarding magnitude of disease load and types of
disease problems in community in terms of morbidity
and mortality
â–ŞBackground data for planning, organizing and
evaluating preventive and curative services
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41. â–ŞEpidemiological methods (Classification)
â–ŞDescriptive epidemiology
â–ŞDefining the population
â–ŞDefining the disease under study
â–ŞDescribing the disease
â–ŞMeasurement of disease
â–ŞComparing with known indices
â–ŞFormulation of etiological hypothesis
â–ŞUses
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Steps