Descriptive epidemiology involves systematically studying the occurrence and distribution of disease in populations. It describes patterns of disease by person, place, and time. Descriptive studies are the first step in epidemiological research as they observe disease occurrence and distribution without inferring causation. They provide basic data on disease frequency and characteristics in a population.

1. DescriptiveDescriptive
EpidemiologyEpidemiology
NAVAS VADAKKANGARANAVAS VADAKKANGARA
33rdrd
SEM MSc MICROBIOLOGYSEM MSc MICROBIOLOGY
DEPARTMENT OF LIFESCIENCESDEPARTMENT OF LIFESCIENCES
UNIVERSITY OF CALICUTUNIVERSITY OF CALICUT 1

2. Definition of Epidemiology*
"The STUDY of the DISTRIBUTION and
DETERMINANTS of HEALTH-
RELATED STATES in specified
POPULATIONS, and the application of this
study to CONTROL of health problems."
*Last, J.M. 1988. A Dictionary of Epidemiology, 2nd ed.
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3. Epidemiology: DefinitionEpidemiology: Definition
Dynamic study of the
Determinants
Occurrence
Distribution
Control
Pattern
Of health and disease in a population
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4. So, EpidemiologySo, Epidemiology
 Is theIs the basic sciencebasic science of public healthof public health
 Provides insight regarding theProvides insight regarding the naturenature,, causescauses,,
andand extentextent of health and diseaseof health and disease
 Provides information needed toProvides information needed to planplan andand targettarget
resourcesresources appropriatelyappropriately
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5. Kinds of EpidemiologyKinds of Epidemiology
 DescriptiveDescriptive
 AnalyticAnalytic
 ExperimentalExperimental
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
Study of the occurrence and
distribution of disease
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6. Descriptive vs. Analytic EpidemiologyDescriptive vs. Analytic Epidemiology
DescriptiveDescriptive
 Used when little isUsed when little is
known about theknown about the
diseasedisease
 Rely on preexistingRely on preexisting
datadata
 Who, where, whenWho, where, when
 Illustrates potentialIllustrates potential
associationsassociations
AnalyticAnalytic
 Used when insight aboutUsed when insight about
various aspects of disease isvarious aspects of disease is
availableavailable
 Rely on development of newRely on development of new
datadata
 WhyWhy
 Evaluates the causality ofEvaluates the causality of
associationsassociations
Both are 6

7. Descriptive StudiesDescriptive Studies
 First step in an epidemiological studyFirst step in an epidemiological study
 Observing a health related event in a populationObserving a health related event in a population
and identifying its characteristicsand identifying its characteristics
 Relatively inexpensive and less time-consumingRelatively inexpensive and less time-consuming
than analytic studies, they describe,than analytic studies, they describe,
 Patterns of disease occurrence, in terms of:Patterns of disease occurrence, in terms of:
 Who gets sick and/or who does notWho gets sick and/or who does not
 Where rates are highest and lowestWhere rates are highest and lowest
 Temporal patterns of diseaseTemporal patterns of disease
 Data provided are useful for:Data provided are useful for:
 Public health administrators (for allocation of resources)Public health administrators (for allocation of resources)
 Epidemiologists (first step in risk factor determination)Epidemiologists (first step in risk factor determination)
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8. Steps in Descriptive StudiesSteps in Descriptive Studies
 Defining population to be studiedDefining population to be studied
 Defining the disease under studyDefining the disease under study
 Describing the disease byDescribing the disease by
 TimeTime
 PlacePlace
 PersonPerson
 Measurement of diseaseMeasurement of disease
 Comparing with known indicesComparing with known indices
 Formulation of an aetiological hypothesisFormulation of an aetiological hypothesis
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9. i)Defining The Populationi)Defining The Population
 Descriptive studies are done on populationDescriptive studies are done on population
NOT on individualsNOT on individuals
 Not only in number, but its composition- Age,Not only in number, but its composition- Age,
sex, occupation, culture etcsex, occupation, culture etc
 Can be a whole population in a geographic areaCan be a whole population in a geographic area
or a representing sample like age groups, sexor a representing sample like age groups, sex
 Should be large enough to be meaningful in theShould be large enough to be meaningful in the
studystudy
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10. ii)Defining the disease underii)Defining the disease under
studystudy
 Needs a definition that enables both valid andNeeds a definition that enables both valid and
precise meaning to obtain an accurate estimateprecise meaning to obtain an accurate estimate
of the conditionof the condition
 Should be identified and measured in theShould be identified and measured in the
defined population with a degree of accuracydefined population with a degree of accuracy
 An “operational definition” for a particularAn “operational definition” for a particular
disease condition may be needed for a defineddisease condition may be needed for a defined
communitycommunity
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11. iii)Describing the diseaseiii)Describing the disease
 The primary objective of the Descriptive study
 Systematic collection and analysis of data
 Study of the occurrence and distribution of
disease in terms of:
 Time
 Place
 Person
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12. What are the three categories ofWhat are the three categories of
descriptive epidemiologic clues?descriptive epidemiologic clues?
 □□ Person:Person: WhoWho is getting sick?is getting sick?
 □□ Place:Place: WhereWhere is the sickness occurring?is the sickness occurring?
 □□ Time:Time: WhenWhen is the sickness occurring?is the sickness occurring?
 PPT = person, place, timePPT = person, place, time
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13. І-TimeІ-Time
 Pattern of the disease occurrence by its time
distribution
 Short-term fluctuations
 Periodic fluctuations
 Long-termsecular trends
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14. aa)-Short term fluctuations)-Short term fluctuations
 The best known is “epidemic”The best known is “epidemic”
 ““The occurrence an illness or other health related-The occurrence an illness or other health related-
events in a community or region clearly in excess ofevents in a community or region clearly in excess of
normal expectancy”normal expectancy”
 The community or region and time period is specifiedThe community or region and time period is specified
preciselyprecisely
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15. Types of Short-term fluctuationsTypes of Short-term fluctuations
(Epidemics)(Epidemics)
 Types of epidemics:Types of epidemics:
 Common source epidemicsCommon source epidemics
 Single exposure or point source epidemicsSingle exposure or point source epidemics
 Continuous or multiple exposure epidemicsContinuous or multiple exposure epidemics
 Propagated epidemicsPropagated epidemics
 Person to personPerson to person
 Arthropod vectorArthropod vector
 Animal reservoirAnimal reservoir
 Slow epidemicsSlow epidemics
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16. Uses of Epidemic curveUses of Epidemic curve
 A graph of the time distribution of epidemicA graph of the time distribution of epidemic
casescases
 It suggest:It suggest:
 A time relationship with exposure to a suspectedA time relationship with exposure to a suspected
sourcesource
 A cyclical or seasonal pattern suggestive of aA cyclical or seasonal pattern suggestive of a
particular infectionparticular infection
 A common source or a propagated spread of theA common source or a propagated spread of the
diseasedisease
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17. l-Common source epidemicsl-Common source epidemics
 The exposure to the disease causing agent isThe exposure to the disease causing agent is
brief and essentially simultaneous, the resultantbrief and essentially simultaneous, the resultant
cases all will develop within one incubationcases all will develop within one incubation
period of the diseaseperiod of the disease
 A point source outbreak curve has usually oneA point source outbreak curve has usually one
peakpeak
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18. Common source, point exposure epidemicCommon source, point exposure epidemic
curvecurve
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19. Point source, Continuous or multiplePoint source, Continuous or multiple
exposure epidemicsexposure epidemics
 Sometimes, a point source can lead to aSometimes, a point source can lead to a
prolonged , continuous, repeated or intermittentprolonged , continuous, repeated or intermittent
exposure – not necessarily at the same time orexposure – not necessarily at the same time or
placeplace
 A point source may can also be give rise to aA point source may can also be give rise to a
propagated epidemicspropagated epidemics
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20. Point source, Continuous or multiple exposurePoint source, Continuous or multiple exposure
epidemic curveepidemic curve
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21. 2-Propagated Epidemics2-Propagated Epidemics
 It is most often an infectious origin and resultsIt is most often an infectious origin and results
from person to person transmission of the agentfrom person to person transmission of the agent
 It shows a gradual rise and tails off over a muchIt shows a gradual rise and tails off over a much
longer period of timelonger period of time
 Transmission continued until number ofTransmission continued until number of
susceptibles depletedsusceptibles depleted
 The speed of spread depend on Herd immunity,The speed of spread depend on Herd immunity,
opportunities for contact and secondary attack rateopportunities for contact and secondary attack rate
 Eg: Epidemics of Hepatitis A, PolioEg: Epidemics of Hepatitis A, Polio
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22. Propagated Epidemics curvePropagated Epidemics curve
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23. b)- Periodic fluctuationsb)- Periodic fluctuations
 Seasonal Trends:Seasonal Trends:
 Seasonal variations are a well known characteristic ofSeasonal variations are a well known characteristic of
many communicable diseasesmany communicable diseases
 Eg: Measles is at its heights during early springEg: Measles is at its heights during early spring
 Related to environmental factorsRelated to environmental factors
 Non-infectious cases also have seasonal trend: sunNon-infectious cases also have seasonal trend: sun
stroke, snake bitestroke, snake bite
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24. 24

25.  Cyclic Trends:Cyclic Trends:
 Diseases occur in cycles spread over short periods ofDiseases occur in cycles spread over short periods of
timetime
 Eg: measles in pre-vaccination era- major peaksEg: measles in pre-vaccination era- major peaks
every 2-3 years, Rubella in every 6-9 yearsevery 2-3 years, Rubella in every 6-9 years
 Eg: Non-infectious conditions: AutomobileEg: Non-infectious conditions: Automobile
accidents are more in rainy months of yearaccidents are more in rainy months of year
 A knowledge of cyclicity is helpful in defendingA knowledge of cyclicity is helpful in defending
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26. C) Long term/Secular TrendC) Long term/Secular Trend
-The long-time trend of disease occurrence
- Progressive increase or decrease
- Several years or decades
- Even if a short term fluctuation may be imposed, it
follow a consistent tendency to change in
particular direction/definite movement in one
direction
- CHD, Lung cancer, Diabetes
- Tuberculosis, Typhoid fever, Polio
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27. Trend of Coronary Heart DiseaseTrend of Coronary Heart Disease
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28. ІІІІ-Place-Place
 Studies of the geography of the disease isStudies of the geography of the disease is
importantimportant
 Difference in disease patterns betweenDifference in disease patterns between
populations, countries, groups in locationspopulations, countries, groups in locations
 Cultures, standards of living, genetic andCultures, standards of living, genetic and
environmental factors, socio-economic factorsenvironmental factors, socio-economic factors
are influences the prevalence of a diseaseare influences the prevalence of a disease
 The variations based on geographic patterns:The variations based on geographic patterns:
 International, National, Urban-rural, LocalInternational, National, Urban-rural, Local
distributiondistribution
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29.  International variations:International variations:
 Eg: cancer exist all over the world, but a markedEg: cancer exist all over the world, but a marked
difference of each cancer in different countriesdifference of each cancer in different countries
 This may help to identify the cause-effectThis may help to identify the cause-effect
relationships of disease and environmental factorsrelationships of disease and environmental factors
 National variations:National variations:
 Disease occurrence may exist within the boundariesDisease occurrence may exist within the boundaries
of countriesof countries
 Endemic goitre, Malaria etc are prevalent in IndiaEndemic goitre, Malaria etc are prevalent in India
and particularly in some partsand particularly in some parts
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30.  Rural- Urban variations:Rural- Urban variations:
 Chronic bronchitis, lung cancer, accidents are moreChronic bronchitis, lung cancer, accidents are more
in urban areas than ruralin urban areas than rural
 Skin and zoonotic diseases, soil transmittedSkin and zoonotic diseases, soil transmitted
Helminths diseases are more in rural areasHelminths diseases are more in rural areas
 Local distributions:Local distributions:
 Inner or outer city disease variations are well knownInner or outer city disease variations are well known
 These are usually mapped with the help of “SpotThese are usually mapped with the help of “Spot
maps” or “Shaded maps”maps” or “Shaded maps”
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31. Role of spot mapsRole of spot maps
 Inner city or outer city variations can be wellInner city or outer city variations can be well
shownshown
 Areas of low-high frequency, boundaries andAreas of low-high frequency, boundaries and
patterns of distribution can be shownpatterns of distribution can be shown
 If map showing a “clustering” of cases, It mayIf map showing a “clustering” of cases, It may
suggest the source of infection or a commonsuggest the source of infection or a common
risk factorrisk factor
 First used by John snow, in London GoldenFirst used by John snow, in London Golden
square Cholera outbreak (in 1854)square Cholera outbreak (in 1854)
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32. 32

33. ІІІ-PersonІІІ-Person
Age Hobbies
Sex Pets
Occupation Travel
Immunization status Personal Habits
Underlying disease Stress
Medication Family unit
Nutritional status School
Socioeconomic factors Genetics
Crowding Religion
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34. iv)Measurement of diseaseiv)Measurement of disease
 To get a picture of “Disease load”To get a picture of “Disease load”
 Information in terms of Mortality, Morbidity,Information in terms of Mortality, Morbidity,
DisabilityDisability
 In Descriptive epidemiology, to measureIn Descriptive epidemiology, to measure
magnitude of disease, use 2 methodsmagnitude of disease, use 2 methods
 Cross-sectional studiesCross-sectional studies
 Longitudinal studiesLongitudinal studies
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35. Cross-sectional studiesCross-sectional studies
 Single examination of a cross section of aSingle examination of a cross section of a
population at one point in time, and resultpopulation at one point in time, and result
projected on the whole populationprojected on the whole population
 Useful in chronic than in acuteUseful in chronic than in acute
 It gives information about the prevalence of aIt gives information about the prevalence of a
disease and only gives a very little informationdisease and only gives a very little information
 It can provide some insights to a later analyticalIt can provide some insights to a later analytical
study, if any may be conductedstudy, if any may be conducted
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36. Longitudinal studiesLongitudinal studies
 Observations are repeated in the same populationObservations are repeated in the same population
over long period of time by means of follow upover long period of time by means of follow up
examinationsexaminations
 It is helpful inIt is helpful in
 To study the natural history of the disease and its futureTo study the natural history of the disease and its future
outcomeoutcome
 For identifying risk factors of the diseaseFor identifying risk factors of the disease
 To find the rate of incidence and frequency ofTo find the rate of incidence and frequency of
occurrence of new cases of diseaseoccurrence of new cases of disease
 It is difficult to conduct and time consumingIt is difficult to conduct and time consuming
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37. v)Comparing with known indicesv)Comparing with known indices
 The essence of epidemiology is comparison andThe essence of epidemiology is comparison and
ask questionsask questions
 It is possible to arrive at clues of diseaseIt is possible to arrive at clues of disease
aetiology by comparing between;aetiology by comparing between;
 Different populationsDifferent populations
 Subgroups in the same populationsSubgroups in the same populations
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38. vi)Formulation of a Hypothesisvi)Formulation of a Hypothesis
 Its a supposition arrived at from observations orIts a supposition arrived at from observations or
reflection, can be accepted or rejected using thereflection, can be accepted or rejected using the
techniques of analytical epidemiologytechniques of analytical epidemiology
 It should be formulated in way that its elementsIt should be formulated in way that its elements
can be subject to be testedcan be subject to be tested
 An improved formulation of hypothesis alwaysAn improved formulation of hypothesis always
needed data to support the fact it mentionneeded data to support the fact it mention
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39. An epidemiologic hypothesisAn epidemiologic hypothesis
must specify:must specify:
 The population- the characteristics of personsThe population- the characteristics of persons
 The specific cause being consideredThe specific cause being considered
 The expected outcome- the diseaseThe expected outcome- the disease
 The dose-response relationship- the amount of the causeThe dose-response relationship- the amount of the cause
needed to lead to a stated incidence of the effectneeded to lead to a stated incidence of the effect
 The time response relationship- The time period elapseThe time response relationship- The time period elapse
between the exposure to the cause and observation of thebetween the exposure to the cause and observation of the
effecteffect
““Cigarette smoking cause lung cancerCigarette smoking cause lung cancer” vs. “” vs. “The smoking ofThe smoking of
30-40 cigarettes per day cause lung cancer in 10 percent of30-40 cigarettes per day cause lung cancer in 10 percent of
smokers after 20 years of exposuresmokers after 20 years of exposure””
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40. Sources of informationSources of information
 Census dataCensus data
 Vital statistical recordsVital statistical records
 Employment health examinationsEmployment health examinations
 Clinical records from hospitalsClinical records from hospitals
 National figures on food consumption ,National figures on food consumption ,
medications, health events etcmedications, health events etc
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41. Uses of Descriptive EpidemiologyUses of Descriptive Epidemiology
 To provide data regarding the magnitude of diseaseTo provide data regarding the magnitude of disease
loadload
 To evaluate trends in health and disease and allowTo evaluate trends in health and disease and allow
comparisons among countries and subgroups withincomparisons among countries and subgroups within
countriescountries
 To provide a basis for planning, provision andTo provide a basis for planning, provision and
evaluation of servicesevaluation of services
 To identify problems to be studied by analytic methodsTo identify problems to be studied by analytic methods
and to test hypotheses related to those problemsand to test hypotheses related to those problems
 To provide clues to disease aetiology and help in theTo provide clues to disease aetiology and help in the
formulation of aetiological hypothesisformulation of aetiological hypothesis
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42. THANK YOUTHANK YOU
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