• Epidemiology : ( comes from a Greek epidemic
meaning upon the people)
“Epi” –among, “demio”-population, “logy”-
• The study of distribution and determinants of
health related states or events in specified
populations and the application of this study
to the prevention and control of health
Distribution: Time, Place, Person
Determinants: Causes or risk factors
Frequency: Incidence, Prevalence
Distribution of disease:
Time- seasonal(e.g.URTI in winter)
Place- geographical( Kala azar in UP,BIHAR,WB
Person- age(measles in 6month to 3 years, RF
in 5to 15 years, cataract in>55yrs)
-sex (migrain, DM in females, RTA in
Epidemiological Purposes in Public
• Discover the agent, host and environmental
factors that affect health.
• Determine the relative importance of causes
(risk factors) of illness, disability and death.
• Evaluate the effectiveness of health programs
and services in improving population health.
Classification of Epidemiological
1) Observational studies : The investigator
measures but do not intervene.
Descriptive study :
• It is a first step in an Epidemiological Studies.
• Defining the population(Unit of study is entire
• Defining the disease (Inclusion , exclusion and
• Description of the occurrence of a disease in a
population in terms of time, place ,person.
• Measurements of disease.
• Comparing with known indices.
• This study helps to generate the hypothesis.
(Hypothesis: It is a logical supposition, a
reasonable guess. Assumption without proof)
Analytical study :
• Second major type
• This study analyses the relationship between
health status and other variables.
• Unit of study is individual.
• This study helps to test the hypothesis.
2) Experimental study : aka Intervention study
• This study confirm the hypothesis.
• It involves an active attempt to change a
disease determinant such as an exposure or a
behaviour or the progress of a disease
• These studies are similar in design to
experiments in other sciences.
1)CROSS SECTIONAL STUDY
• Aka prevalence study.
• It is simplest form of an observational study.
• Unit of study is individual.
• It is based on a single examination of cross
section of population at one point in time so
the results of which can be projected on
• In this study, measurements of exposure and
effects(outcome) are made at the same time,
so this study is useful for investigating the
• Study is similar to a survey in such a way that,
it provides a snapshot of the population at a
point in time. So also called as snap chat
• Using this study, the epidemiologist defines
the target population, then collects data from
the population or a subset of the population
at one specific point in time.
• Participants are selected regardless of their
exposure or disease status.
• This study useful in sudden outbreak of
disease and for chronic diseases.
Cross sectional study continued……..
• For example ,in a study of prevalence of
hypertension, we collect data during survey
about age, gender, body weight, physical
exercise, salt intake and other variables.
• Study helps in assessing load of disease, needs,
resources, patterns of health utilization,
practices and knowledge attitudes.
• This study tells us about the distribution of a
disease in a population rather than its etiology.
• Relatively quick and easy to conduct, no follow
• Data related to all variables is collected at
• Study includes multiple outcomes and
• It is a best way to understand disease load and
plan health services.
• Generation of hypothesis.
• It is difficult to find out whether the outcome
followed exposure in time or it is resulted due to
• Not suitable for rare disease or disease with
• Study measures prevalent cases( new and old
cases) rather than incident cases so we can’t
• The associations obtained in this study would be
difficult to interpret.
2)CASE CONTROL STUDY
• Aka case referent or retrospective study.
• This is the first approach to test a casual
• Study Disease > Risk factor ( Backward looking)
as the Investigator is looking backwards from the
disease to a possible cause.
• In this study, subjects are selected on the basis of
whether they do (cases) or do not (controls) have
• The groups are compared with respect to the
proportion having a history of an exposure .
Framework of case control study
Suspected or risk
Cases (Lung Cancer) Controls (No lung
Present a b
Absent c d
Total a+c b+d
Steps in Case Control Study
1) Selection of Case – A case is someone who has
disease under investigation.
• It should be objective in nature and should be followed
meticulously throughout the study.
• Cases should represent a specified population group on
the basis of disease, not exposure. For this one needs
diagnostic criteria of that particular disease and also
stage of the disease
• The Second criteria is related to eligibility. The old
cases or cases that have advanced stage of the disease
may not be eligible for inclusion in the study.
• Sources of cases :
1. Hospitals: From a single hospital or network
of hospitals either case series or a random
sample, admitted during a specified period of
time. ( convenient)
2. General population: selected in a
geographical area through survey, a disease
registry or hospital network.
2) Selection of control
• Control is someone who doesn’t have the
disease under investigation.
• Control should be similar to the cases as
possible except for the disease under study
• So controls should be indentified before the
• Sources: controls from Hospital, relatives ,
neighbors, occupational associates or general
• The controls may differ from the cases in a
number of factors such as age, sex, occupation,
education, social status etc.
• To share the comparability between cases and
controls matching is done.
• It is the process, by which select controls in such
a way that they are similar to cases with regard to
certain pertinent variables, which are known to
influence the outcome of the disease.
• If Matching not done ,this could distort or
confound the results.
• Confounding factor – Is defined as one which
is associated with both exposure and disease
and is distributed unequally in cases and
• Precautions while matching
- matching factor should be associated with
disease not on exposure
4)Measurements of exposure
• In this study, the exposure status of the cases is usually
determined after the development of the disease and
by Questionnaire, physical examination, laboratory
investigations and study of records.
• The exposure may be of various types.(e.g. habit of
alcohol drinking, smoking)
• The measurement of exposure can be done by various
methods. Such as estimation of causative agent in
environment (carbon monoxide in air), estimation of
pathological changes(RBC stripling,Hb level in lead
• Final step in analysis, done to find out
(a) Exposure rates among cases and controls to
suspected factor : provides direct estimation
of exposure rates(frequency of exposure) to a
suspected factor in disease and non disease.
(b)Elimination of disease risk associated with
Smoking Lung Cancer Not having Lung
Exposure present 90(a) 20(b) 110
Exposure absent 10(c) 80(d) 90
Total 100 100 200
• Exposure rates:
Cases = a /(a+c) = 90/100
Controls = b /(b+d) = 20/100
This shows that, frequency of lung cancer was
higher among smokers than among non
Estimation of Risk
• It is obtained by Relative Risk(RR) or risk ratio is
defined as the ratio between the incidence of
disease among exposed persons and incidence
among non exposed.
Incidence among exposed
Relative risk =
Incidence among nonexposed
= a /(a+c) ÷ b /(b+d)
• A typical case control study, does not provide
incidence rate from which relative risk can be
• Next step Is to estimate the risk of disease
associated with the exposure Odds ratio
(Cross product ratio) : Determine the
association of an exposure and a disease.
• It is similar to risk ratio
Derivation of odds ratio is on 3 assumptions:
• The disease under investigation must be
• The cases must be representative of those
with the disease
• The controls must be free from disease.
• Odds ratio = ( a/ b)
( c/ d )
= ad / bc
= 90* 80/ 20* 10
= 7200 / 200
Odds ratio is 1 , no association
Odds ratio is >1 , positive association
Odds ratio is <1 , negative association
In the above example, people who smoke(cases)
were having a risk of developing lung cancer
36 times that of non smokers.
• It is any systematic error in the determination
of the association between exposure and the
a) Bias due to confounding
b) Memory or recall bias
c) Selection bias
d) Berkesonian bias
e) Interviewers bias
• Easy to conduct, consume less time,
• Suitable for rare disease.
• There is no risk to the subjects as disease has
• It is possible to study different causative
factors responsible for a particular disease.
• Minimal ethical problem
• Study depends upon on
• It is very difficult to get a perfect control.
• It is not possible to measure incidence and
• This study can not distinguish between causes
and associated factors
3) COHORT STUDY
• Cohort studies also called as Follow up ,
Incidence study, Prospective study, Forward
looking study or longitudinal study.
• Cohort study is similar to a longitudinal
descriptive study with exception that there is
comparison group in cohort.
• Cohort is defined as a group of people who
share a common characteristics or exposure
within a specified time period (Birth cohort).
• Cohort studies are indicated when there is good
evidence of association between exposure and
when exposure is rare and incidence is high.
• They are indicated when follow up is easy, cohort
is stable, cooperative and easily accessible and
ample funds are available.
• Cohorts must be free from the disease under
study and then they are classified into subgroups
according to exposure to a potential cause of
disease or outcome. And followed up for study
under same identical conditions.
• Both groups should be comparable in respect
of all the possible variables, which may
influence the frequency of disease.
• Diagnostic and eligibility criteria of disease
must be defined before the study.
Types of cohort
Steps of a cohort study
1) Selection of study subjects
a) General Population: people residing in well
defined geographical area.
b) Special groups: such as professional group,
governments employees, volunteers, etc.
these groups are usually homogeneous
2) Obtaining data on exposure: data collected
through personal interviews, mailed
questionnaires, previous records, medical
examination or special tests.
• Groups are divided according to whether they
have been exposed to a suspected factor
(case) or not exposed (control) if yes then
divided again as per degree of exposure
3) Selection of comparison group:
Internal comparison: In some study no outside
comparison group required. On the basis of
information obtained single cohort group are
classified into several comparison groups
according to degree or level of exposure to risk
before development of disease.(e.g. smoking,
External comparison: when degree or level of
exposure is not availble.
Comparison with general population: If none
available or the difficulties of selecting the
study and comparison group.
4) Follow up: It is a most important step. To obtain
data for assessing the outcome, the procedures
A. Periodical medical examination of each member
of cohort as it provides more information.
B. Reviewing records from physicians and hospitals.
C. By surveillance of routine death records.
D. Mailed questionnaire, telephone calls and
periodic home visits on annual basis.
5) Analysis: In this study, we analysis data by
calculating incidence rates of disease for both
cohorts, and then estimating relative risk,
attributable risk and population attributable
Disease present Disease absent Total
Cause present a (30) b (70) a+b (100)
Cause absent c (10) d (90) c+d (100)
1. Relative Risk: It is defined as the ratio
between the incidence of disease among
exposed persons and incidence among non
• In above example,
Incidence rate among smokers is=30/100
Incidence rate among non smokers is=10/100
• Therefore, Relative risk = 30/10 = 3.
• Relative risk has etiological enquiries as it
provides direct measure of the strength of
association between suspected cause or
exposure and effect.
• In this example, relative risk is 3 means people
who smoke cigarettes, have 3 times more risk
of developing lung cancers than non smokers.
2. Attributable risk/Risk difference: It is the
difference in incidence rates of disease
between exposed group and non exposed
• It provides info. about the contribution of
suspected cause for the occurrence of disease.
(or how much of the disease can be attributed
to suspected cause.)
(Incidence of disease in exposed –
Incidence of disease in non exposed × 100
Incidence rate among exposed
• In this example,
AR= 0.3-0.1 × 100
= 0.2/0.1 × 100
• So in this example, It shows that, 66% of the lung
cancers among smokers could be attributed to
smoking. And hence there is a causal association.
• This information helps in knowing how much
of the disease can be eliminated by
eliminating or controlling the suspected cause
3. Population attributable risk: It is the
incidence of the disease (or death) in the total
population minus incidence of the disease (or
death) among those who were not exposed to
the suspected causal factor.
• It provides an estimate of the amount by
which the disease could be reduced in that
population if the suspected cause or exposure
was eliminated or controlled.
• We find out incidence.
• The beginning point is cohort with and
without exposure, so from this we can study
many outcomes related to exposure.
• Relative risk is being calculated.
• Minimal bias
• Unsuitable for rare diseases.
• Time consuming. (migration, death ,behaviour
change or loss of interest, population attrition)
• Administrative problems( lack of experienced
staff, shortage of fund).
• Over a period of time, the diagnostic criteria or
definition of a disease under study may change.
• Selection of comparison groups which are
representative of the exposed and unexposed
segments of the population is a limiting factor.
• Ethical issues.
Nested case control study
• It is a case control study “nested” within a
prospective or retrospective cohort study.
• In this study design, Cases and Controls both
are from cohort sample.
• It is useful for predictor variables that are
expensive, useful for costly analysis of
specimens, like biochemical analysis of serum
samples that are taken at the beginning of the
study and then preserved for later analysis.
• STEPS :
• Identify a cohort with a pool of specimens
collected at baseline.
• Identify those developing disease during
• Select the sample from cohort who do not
develop the disease(Control)
• Measure the predictor variables.
• Aka correlational study
• Useful for generating hypothesis
• Units of analysis are groups of people
• These studies can be done by comparing
populations in different places at same time or
in a time series or comparing same population
at one place at different time.
• As it is easy to conduct, attractive, these study
are often difficult to interpret.
• Ecological studies usually rely on data
collected for other purposes.
• The association between exposure and effect
at the individual level cant be made.
• Ecological fallacy: bias results from if
inappropriate conclusions are drawn on the
basis of ecological data.
Surveillance: The ongoing systemic collection, analysis and interpretation of health data essential to the planning, implementation and evaluation of public health practice,closely integrated with the timely dissemination of these data to those who need to know. [ centers for disease control and prevention proposed by Langmuir 1963]
Hypothesis: . It provides a tentative explanation for a phenomenon under investigation
that are fixed characteristics of individuals such as ethnicity or blood group. (The study involves looking at people who differ on one key characteristic at one specific point in time.)
Case Control studies are longitudinal.
Data : will be collected from hospital ,workplace or general population
Pertinent – getting attached/affiliated
easily accessible which minimizes the problem of attrition of study population.
Term prospective refers to the timing of data collection
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