Community Medicine/Public Health

1. Epidemiology
Dr.Shahzad A.Daula
MID (UOL) M.Phil. Public Health (Pb)

2. Defnition
Study of the distribution, dynamics
and determinants of a disease in a
human population is known as
Epidemiology.

3. CLASSIFICATION OF EPIDEMIOLOGIC
METHODS
Epidemiologic
methods
Observational
studies
Descriptive
studies
Analytical
studies
Ecological
Cross
sectional
Case control
cohort
Experimental or
Interventional
studies
Randomized
controlled trials
Field trials
Community
trials 2

4. Descriptive Epidemiology
In Descriptive epidemiology we organize &
analyze the data to understand variation in
disease frequency

5. PROCEDURES IN DESCRIPTIVE STUDIES
1) Defining the population to be studied
2) Defining the disease under study
3) Describing the disease by
4) Measurement of disease
5) Comparing with known indices
6) Formulation of an aetiological hypothesis
a) Time
b) Place
c) Person

6. 1) TIME DISTRIBUTION
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
1
0

7. 2) PLACE DISTRIBUTION
Place
variations
1) International
variation
2) National
variation
3) Rural-Urban
variation
4) Local distribution 25

8. 3) PERSON DISTRIBUTION
8
The disease can be characterised by defining a
person who develops a disease based on
age, sex,
occupation, marital status,
social factors, habits

9. Analytical Epidemiology
Use to quantify the association between
exposures and outcomes and to test hypotheses
about causal relationships.

10. Case Control Study
 Both exposure and outcome have occurred before
the onset of the study.
 Study proceeds backwards, from effect to cause.
 Use a control or comparison group to support or
refute an inference.

11. Controls
Not exposed
Designof acase-control study
Time
Direction of enquiry
Exposed
Cases
Not exposed
Population
Exposed
Retrospective study

12. Stepsin …
1. Selectionof casesandcontrols
 Eligibility Criteria
 Sources of cases and Control
 Similarity between cases & Control except Disease
2. Matching
 Groupmatching
 Matching bypairs
3. Measurement of exposure
 Interviews Questionnaires By studying past records Examination
4. Analysisandinterpretation
 Exposure rate among case & control to suspected factor
 Estimation of diseaserisk associatedwith exposure(oddsratio)
… Case- Control Studies

13. Advantages
• Relativelyeasy to carryout
• Rapid and inexpensive
• Require comparatively few subjects
• suitable to investigate rare diseases.
• Ethical problems minimal
• Allows the study of severaldifferent etiological factors
• Ethical problems minimal
• studies do not require follow-up of individuals into the future

14. Disadvantages
• Highchancesfor bias.
• Selection of an appropriate control group may
be difficult
•
•
•
•
•
• We can not measure incidence, and can only
estimate the odds ratio
• Not suited to the evaluation of therapy or prophylaxis of a
disease
… Case- Control Studies

15. Cohort study
1. Cohorts are identified prior tothe appearanceof the disease
underinvestigation
2. Studygroups are observed over aperiod oftime to determine
the incidence ofdisease
3. Thestudy proceeds from causeto effect
… Cohort studies

16. • Cohort is defined as a group of people who share a common
characteristic or experience within a defined time period
• Eg. age cohorts, occupational cohorts, exposure to a drug cohorts,
marriage cohortetc.

17. Design of acohort study
Time
Direction of enquiry
Disease
Exposed
No disease
Healthy
Population
Disease
Not exposed
No disease
… Cohort studies

18. Stepsin …
• Selectionof studycohorts
 General Groups
 Special Groups
• Obtaining data onexposure
 Cohortmembers
 Reviewofrecords
 Examinationandsurveys
• Selectionof comparisongroups
 Internalcomparison
 Externalcomparisons
 Comparisonwith generalpopulation
• Followupandanalysis
… Cohort studies

19. Followup
•Periodic medical examination of each member
•Reviewing physician and hospital records
•Routine surveillence of morbidity and
mortality records
•Mailed questionnaires, telephone interviews,
periodic home visits
… Cohort studies

20. Analysis
•Datais analysed intermsof
i. Incidence rates of outcomeamong exposedandnon-exposed
Estimation of risk
•
•
Relative risk
Attributable risk
… Cohort studies

21. Advantages
 Allow the possibility of measuring directly the relative risk of developing the
condition for those who have the characteristic, compared to those who do
not.
 Allowsfor aconclusion of cause-effectrelationship.
 Becausethe presence or absence of the risk factor is recorded before the disease
occurs, there is no chance of bias being introduced due to awareness of being
sick asencountered in case-controlstudies.

22.  Dose-response ratios canbecalculated.
 Cohort studies are capableof identifying other diseasesthat mayberelated
to the sameriskfactor.
 Biascanbe avoided.
 Unlike case-control studies, cohort studiesprovide the possibility of
estimating attributable risks, thus indicating the absolute magnitude of
disease attributable to the riskfactor.

23. Disadvantages
-Not alwaysfeasible.
-Relatively inefficient for studyingrare conditions.
-Samplesizesrequired for cohort studiesare extremely large,
especially for infrequent
conditions; it is usually difficult to find and managesamples of
this size.
-Verycostly

24. Case control study Cohort study
Proceeds from effect to cause Proceeds from "cause to effect"
Starts with the disease Starts with people exposed to risk factor or
suspected cause.
Tests whether the suspected cause
occurs more frequently in those with
the disease than among those without
the disease.
Tests whether disease occurs more
frequently in those exposed, than in
those not similarly exposed.
Usually the first approach to the
testing of a hypothesis, but also useful
for exploratory studies
Reserved for testing of precisely
formulated hypothesis
Involves fewer number of subjects Involves larger number of subjects

25. Yields relatively quick results Long follow-up period often needed,
involving delayed results.
Suitable for the study of rare diseases Inappropriate when the disease or
exposure under investigation is rare.
Cannot yield information about
diseases other than that
selected for study
Can yield information about more than
one disease outcome.
Relatively inexpensive Expensive.

26. Jzak Allah