Epidemiology is defined as the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to control health problems. The document discusses various types of epidemiological studies including observational studies like descriptive, analytical, and experimental studies like randomized controlled trials. It describes the aims, principles, and tools of measurement in epidemiology. The key analytical study designs of case-control studies and cohort studies are explained in detail, including how they are conducted and their advantages and limitations.

1. ANALYTICALANALYTICAL
EPIDEMIOLOGYEPIDEMIOLOGY
KAUSTUBH SINGH
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2. What is epidemiology ?
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3. Defined by John M. Last in 1988
 “Study of Distribution and Determinants of
health related state or event in a specified
population and the application of this study to the
control of health problem”.
 We measure –
– Disease frequency
– Diseases distribution
– Determinants of disease.
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4. TYPES OF EPIDEMIOLOGICAL STUDIES
 1. OBSERVATIONAL STUDIES
 A. DESCRIPTIVE STUDY
 DESCRIBE DIESEASE BY
 TIME
 PLACE
 PERSON
 B. ANALYTICAL STUDIES
 ECOLOGICAL STUDY
 CROSS SECTIONAL STUDY
 CASE-CONTROL STUDY
 COHORT STUDY
 2. EXPEREMENTAL STUDIES
 RANDOMIZED CONTROLLED TRIAL (RCT)
 FIELD TRIAL
 COMMUNITY TRIAL
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6. AIMS OF EPIDEMIOLOGY
 to describe the size and distribution of the
disease problems in human populations.
 to provide the data essential for the
planning, implemnetation and evaluation
of health services for the prevention,
control and treatment of diseases and for
the setting up of priorities among those
services.
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7.  to identify etiological factors in the
pathogenesis of disease.
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8. PRINCIPLES OF EPIDEMIOLOGY
 exact observation (strict, vigrous,
accurate, precise)
 Correct interpretation( free from error)
 rational explanation (intelligent, sensible ,
reasonable)
 scientific consteuction( by expert
knowledge and technical skill)
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9. TOOLS OF MEASUREMENT
IN EPIDEMIOLOGY
 Rates
 Ratios
 Proportions
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ANALYTICAL
EPIDEMIOLOGY
 Testing a hypothesis
*Statistical association between a disease and suspected
factor
*Strength of association
 The subject of interest is individual within the population
 Inference is to the population from which individuals are
selected

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 They are of 2 types
* Case control study
* Cohort study

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CASE- CONTROL STUDY
 Subjects are selected on the basis of whether they
do (cases) or do not (controls) have a
particular disease under study.
 Compared with respect to proportion having a
history of an exposure or characteristic of
interest.

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Distinct feature
 Both exposure and outcome (disease) have
occurred before the start of study
 The study proceeds backwards from effect to
cause, and is thus called retrospective
 It uses a control or comparison group to support
or refute an inference

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Basic Steps in conducting a Case
Control Study
 Selection of cases and controls
 Matching
 Measurement of exposure
 Analysis and interpretation

16. 1. Selection of Cases and Controls
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 Identify a suitable group of cases and a group of
controls
 Comparability of cases and controls is essential
 Major issues to be considered are:
 ‐ selection of study groups
 ‐ sources of information about exposures and
disease

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18. Selection of cases
 Definition of disease
– Diagnostic criteria-Once the diagnostic criteria are
established, they should not be altered or changed till the study is over.
- Eligibility criteria- it eliminates the possibility that the
long term survivors of disease were exposed to a investigated risk factor
after the onset of disease.
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19. Sources of cases
 Hospital
 General Population
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20. Selection of controls
* Must be
 free from disease under study
 as similar to case as possible
 Comparison group identified before study
is done
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21. Sources of controls
 Hospital Control
 Relatives
 Neighborhood control
 General population
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22. Pancreatic cancer & Coffee
Consumption
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24. Hospital controls
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 Advantages
 Easily identified and readily available
 More aware of antecedent exposures or events, reduce
potential for recall bias
 Likely to have been subjected to the same intangible
selection factor
 More likely to be willing to co operate than healthy‐
individuals, minimizing bias due to non response‐

25.  Disadvantage
– They are ill and therefore differ from healthy
individuals in a number of ways may be
associated with illness and hospitalization in general.
– Select from various diagnostic groups
– Don’t select patient with diagnosis known to
be related to risk factor of interest
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26. General Population
 From defined geographical areas
 Disadvantages:
 ‐ more costly and time consuming
 ‐ difficult to contact
 ‐ quality of information may differ
 ‐ individuals refuse to participate
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27. How many controls are
needed?
 Ideally, 1 case : 1 control
 May be 2 controls per case depending
upon:size of study group
 time
 cost
Multiple controls provide a check on bias
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28. 2. Matching
 A major concern in conducting a case‐
control study is that cases and controls
may differ in characteristics or exposures
other than the one that has been targeted
for study.
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29.  e.g. if most of the cases are poor and most
of the controls are affluent, we would not
know whether the factor determining
development of disease is exposure to the
factor being studied or another
characteristic associated with being poor.
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30.  Matching is defined as the process of selecting
the controls so that they are similar to the cases
in certain characteristics, such as age, race, sex,
socioeconomic status, and occupation.
 If not adequately matched, could distort or
confound the results.
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31. Group Matching
 Group matching (or frequency matching):
– Proportion of controls with a certain
characteristic is identical to the proportion
of cases with the same characteristic. e.g. if
25% of the cases are married, the controls will be
selected so that 25% of that group is also married.
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32. Individual Matching
 Individual matching (or matched pairs).
 A control is selected who is similar to the
case in terms of the specific variable or
variables of concern. e.g., if the first case
enrolled in our study is a 25 year old‐ ‐
female, we will seek a 25 year old‐ ‐
female control.
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33. What are the problems with
matching?
 Practical Problems with Matching:
 Match according to too many
characteristics, it may prove difficult or
impossible to identify an appropriate
control. e.g., match each case for race, sex,
age, marital status, number of children, zip
code of residence, and occupation.
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34.  2. Conceptual Problems with Matching:
– Once we have matched controls to cases
according to a given characteristic, we cannot
study that characteristic. For example, suppose
we are interested in studying marital status as
a risk factor for breast cancer.
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35. 3. Measurement of Exposure
 Definition and criteria for exposure
 Obtain by interviews,
 questionnaire,
 past records etc.
 Must rule out ‘bias’
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36. 4. Analysis
 To find out
 Exposure rates among cases and controls
 Estimation of disease risk associated with
exposure (Odds Ratio)
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37. EXPOSURE RATES
 A case control study provides a direct
estimation of the exposure rates to a
suspected factor in disease and non-
disease groups.
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38. Case Control Study Analysis
CASES CONTROLS
Exposed a b
Non exposed c d
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39.  Exposure rates
* Cases = a/(a+c)
* Controls = b/(b+d)
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40. Estimtion of risk
 “RELATIVE RISK” – defined as the ratio
between the incidence of disease among exposed
persons and incidence among non exposed
 RR = incidence among exposed / incidence
among non exposed
 = {a/(a+b)}/{c/ (c+d}
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41. Odds ratio (Cross product‐
ratio)
Assumptions:
 The disease being studied must be
relatively rare.
 The cases must be representing of those
with the disease.
 The controls must be representing of those
without the disease.
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42.  The odds in the exposed group are
compared with the odds in the unexposed
group:
 (Odds of disease in the exposed group) /
(Odds of disease in the unexposed group)
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43.  Odds of disease in the exposed group: a/b
 Odds of disease in the unexposed group:
c/d
 Odds Ratio = (a/b) / (c/d) = ad/bc
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44. Bias
 Systematic error in the determination of
association between the exposure and
disease.
 The relative risk estimate may increase or
decrease as a result of the bias.
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45. Bias in Case Control Studies
 Bias due to confounding
 Memory/Recall Bias
 Selection Bias
 Interviewer’s Bias
 berkesonian bias
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46. Advantages of Case Control
Studies
 Easy to carry out.
 Suitable to investigate rare diseases.
 No risk to the subjects.
 Allows study of aetiological factors.
 Risk factors can be identified.
 No attrition problems.
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47. Disadvantages of case Control
Studies
 Problems of bias relies on past records.
 Selection of control group is difficult.
 Can't measure incidence.
 Not suited to evaluation of therapy or
prophylaxis of disease.
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49. WHAT IS COHORT
 Ancient Roman
military unit, A band
of warriors.
 Persons banded
together.
 Group of persons with
a common statistical
characteristic. [Latin]
 E.g. age, birth date,
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50. Cohort studiesCohort studies
longitudinallongitudinal
Prospective studiesProspective studies
Forward looking study IForward looking study I
Incidence studyIncidence study
starts with people free of diseasestarts with people free of disease
assesses exposure at “baseline”assesses exposure at “baseline”
assesses disease status at “follow-up”assesses disease status at “follow-up”
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51. INDICATION OF A COHORT
STUDY
 When there is good evidence of exposure
and disease.
 When exposure is rare but incidence of
disease is higher among exposed
 When follow-up is easy, cohort is stable
 When ample funds are available
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54. Frame work of Cohort studiesFrame work of Cohort studies
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56. General consideration while selection ofGeneral consideration while selection of
cohortscohorts
Both the cohorts are free of the disease.Both the cohorts are free of the disease.
Both the groups should equally susceptibleBoth the groups should equally susceptible
to diseaseto disease
Both the groups should be comparableBoth the groups should be comparable
Diagnostic and eligibility criteria for theDiagnostic and eligibility criteria for the
disease should be defined well in advance.disease should be defined well in advance.
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57. Elements of cohort study
 Selection of study subjects
 Obtaining data on exposure
 Selection of comparison group
 Follow up
 Analysis
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58. Selection of study subjects
 General population
– Whole population in an area
– A representative sample
 Special group of population
– Select group
• occupation group / professional group (Dolls study )
– Exposure groups
• Person having exposure to some physical, chemical or
biological agent
– e.g. X-ray exposure to radiologists
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59. Obtaining data on exposure
 Personal interviews / mailed questionnaire
 Reviews of records
– Dose of drug, radiation, type of surgery etc
 Medical examination or special test
– Blood pressure, serum cholesterol
 Environmental survey
 By obtaining the data of exposure we can classify
cohorts as
– Exposed and non exposed and
– By degree exposure we can sub classify cohorts
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60. Selection of comparison group
 Internal comparison
– Only one cohort involved in study
– Sub classified and internal comparison done
 External comparison
– More than one cohort in the study for the purpose of comparison
– e.g. Cohort of radiologist compared with ophthalmologists
 Comparison with general population rates
– If no comparison group is available we can compare the rates of
study cohort with general population.
– Cancer rate of uranium miners with cancer in general population
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61. Follow-up
 To obtain data about outcome to be determined
(morbidity or death)
– Mailed questionnaire, telephone calls, personal
interviews
– Periodic medical examination
– Reviewing records
– Surveillance of death records
– Follow up is the most critical part of the study
 Some loss to follow up is inevitable due to death change
of address, migration, change of occupation.
 Loss to follow-up is one of the draw-back of the cohort
study.
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62. ANALYSIS
 Calculation of incidence rates among
exposed and non exposed groups
 Estimation of risk
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63. Incidence rates of outcome
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64. Incidence rate
 Incidence among exposed =
a
a+b
 Incidence among non-exposed =
c
c+d
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65. Estimation of risk
 Relative Risk
incidence of disease among exposed
RR = ______________________________
Incidence of disease among non-exposed
a/a+b
= c/c+d
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66.  Attributable Risk
Incidence of disease among exposed –
incidence of disease among non exposed
AR = Incidence of disease among exposed
a/a+b – c/c+d
AR = a/a+b
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67. Find out RR and AR for above data
Smoking Lung cancer Total
YES NO
YES 70 6930 7000
NO 3 2997 3000
73 9927 10000
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68.  Incidence of lung cancer among smokers
70/7000 = 10 per 1000
 Incidence of lung cancer among non-smokers
3/3000 = 1 per thousand
RR = 10 / 1 = 10
(lung cancer is 10 times more common among
smokers than non smokers)
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69. AR = 10 – 1 / 10 X 100
= 90 %
(90% of the cases of lung cancer among
smokers are attributed to their habit of
smoking)
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70. Types of Cohort Study
 Prospective cohort study
 Retrospective (historical) cohort study
 Combination of Retrospective and
Prospective cohort study.
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71. Cohort studies
Strengths
 We can find out
incidence rate and
risk
 More than one disease
related to single
exposure
 can establish cause -
effect
 good when exposure
is rare
 minimizes selection
and information bias
Weaknesses
 losses to follow-up
 often requires large sample
 ineffective for rare diseases
 long time to complete
 expensive
 Ethical issues
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