Epidemiology fundamentals

SHABNA.G.S
FIRST YEAR MDS
DEPARTMENT OF PUBLIC HEALTH DENTISTRY
GOVERNMENT DENTAL COLLEGE,KOTTAYAM
FUNDAMENTALS OF EPIDEMIOLOGY
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CONTENTS
INTRODUCTION
DEFINITIONS
AIMS AND OBJECTIVES OF EPIDEMIOLOGY
ULTIMATE AIM OF EPIDEMIOLOGY
TOOLS OF MEASUREMENT OF EPIDEMIOLOGY
MEASUREMENT OF MORTALITY AND MORBIDITY
EPIDEMIOLOGICAL METHODS
CONCLUSION
BIBLIOGRAPHY

Epidemiology is derived from the word epidemic
[epi=among;demos=population;logos=study]
Epidemiology is the basic science of preventive
and social medicine.
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K. Park, Park’s Textbook of preventive and social medicine.18Th edition.
Banarsidas Bhanot Publishers, (2005) 1-2

DEFINITIONS
1.. That branch of medical science which treats
epidemics (Parkin, 1873).
2. The science of the mass phenomena of infectious
diseases (Frost, 1927).
3. The study of disease, any disease, as a mass
phenomenon (Greenwood, 1934).
4. The study of the distribution and determinants of
disease frequency in man (MacMahon, 1960).
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Epidemiology has been defined by John M. Last in
1988 as:-
The study of the distribution and determinants of
health-related states or events in specified
populations, and the application of this study to the
control of health problems.
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K. Park, Park’s Textbook of preventive and social medicine.18 Th edition.

AIMS & OBJECTIVES OF
EPIDEMIOLOGY
1. To describe the distribution and magnitude of
health and disease problems in human population.
2. To identify etiological factors (risk factors) in
the pathogenesis of disease.
3. To provide data essential to the planning,
implementation and evaluation of services for
the prevention, control and treatment of disease
and setting priorities among those services.
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ULTIMATE AIM OF EPIDEMIOLOGY
1. To eliminate or reduce the health problems of
community.
2. To promote the health and well-being of society
as a whole.
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TOOLS OF MEASUREMENT IN EPIDEMIOLOGY
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RATE
 Measures the occurrence of an event or disease in
a given population during a given period (one
Year).(Birth rate, growth rate, accident rate)
 Usually expressed per 100 or per 1000 population.
 It has a time dimension, whereas a PROPORTION
does not.
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RATIO
 The value obtained by dividing one quantity by
another- X/Y.
 Male to female ratio.
 A ratio often compares two rates, death rates for
women and men at a given age.
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 Ratio also expresses relation of size between the
two quantities.
 Numerator is not part of Denominator.
 Expressed as X / Y.
 Male : Female ratio.
 WBC : RBC ratio
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PROPORTION
 Usually expressed as a percentage %
 Numerator (which is part of denominator)
 Denominator
 Multiplier
 No time factor
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MEASUREMENT OF MORTALITY
• Crude death rate
• Specific death rate
• Proportional mortality rate
• Case fatality rate
• Survival rate
• Adjusted/standardized rates
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CRUDE DEATH RATE
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MEASUREMENT OF MORBIDITY
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RELATION BETWEEN INCIDENCE &
PREVALENCE
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RELATION BETWEEN INCIDENCE &
PREVALENCE
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Prevalence = Incidence X duration
• Incidence = 10 cases/1000 population/year
• Mean duration of disease = 5 years
• Prevalence = 10 x 5 = 50 per 1000 population

SPECIAL INCIDENCES
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CUMULATIVE INCIDENCE/ CUMULATIVE
INCIDENCE PROPORTION OR RISK
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New cases accumulated in given period of time
in a population at risk
Reflects the probability of individuals in the
population to get the disease in the given time
period (Risk)
Beaglehole R, Bonita R, Kjellström T. Basic Epidemiology. 2nd ed. Geneva:
World Health Organization; 2006.

TYPES OF PREVALENCE
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PREVALENCE INCREASED BY
Longer duration of the disease.
 Prolongation of life, with treatment.
 If incidence increases.
 Immigration of new cases.
 Better reporting of cases.
 Emigration of healthy people.
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PREVALENCE DECRESED BY
Shorter duration of diseases.
 Improved cure rate.
 Incidence decreases.
 Emigration of new cases.
 Under reporting of cases.
 Immigration of healthy people.
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SAMPLING AND ITS TYPES
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•A sample is a subset of a population that is used to
represent the entire group as a whole.
•When doing research, it is often impractical to
survey every member of a particular population
because the number of people is simply too large.

WHAT IS A 'GOOD' SAMPLE?
 It represents the population under study in the
characteristics of interest.
 It is adequate in numbers.

WHAT IS AN 'ADEQUATE’ SAMPLE?
 The number must not be too few that we can not rule
out chance.
 It should not be too large that it is uneconomical.

SAMPLING
 Probability (random) sampling
 Sampling in which each sampling unit has a
known and equal probability of being included
in the study.
 Non Probability (Non random) sampling.

WHAT IS A 'RANDOM' SAMPLE?
A 'random' sample, strictly, is a sample
generated from a truly random process.

TYPES OF RANDOM SAMPLING
 Simple random sampling
 Systematic random sampling
 Stratified random sampling
 Cluster sampling
 Multi stage Sampling

SIMPLE RANDOM SAMPLING
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• Applicable when population is small, homogeneous
& readily available.
• All subsets of the frame are given an equal
probability. Each element of the frame thus has an
equal probability of selection. A table of random
number or lottery system is used to determine which
units are to be selected.

SYSTEMATIC SAMPLING
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Similar to simple random sample. No table of
random numbers – select directly from sampling
frame.

STRATIFIED SAMPLING
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The population is divided into two or more
groups
called strata, according to some criterion, such
as
geographic location, age, or income, and sub
samples are randomly selected from each strata.

CLUSTER SAMPLING
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 With cluster sampling, the researcher divides the
population into separate groups, called clusters. Then,
a simple random sample of clusters is selected from
the population. The researcher conducts his analysis
on data from the sampled clusters.

MULTISTAGE SAMPLING
 Employed in large country surveys.
 Sample procedures carried out in several stages
using random sampling techniques.
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EPIDEMIOLOGIC RESEARCH METHODS
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Beaglehole R, Bonita R, Kjellström T. Basic Epidemiology. 2nd ed.
Geneva: World Health Organization; 2006.

DESCRIPTIVE STUDY DESIG
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Descriptive design is limited to a description of
the occurrence of a disease in a population.
i.e. identifying the characteristics with which the
disease seems to be associated.
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Types
Case reports
Case series
Cross sectional
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CASE REPORT
A case report is…
“Biomedical story-telling” (Lawrence, 1991)
A stimulant for more comprehensive and prospective
research
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CASE SERIES
Experience of a group of patients with a similar
diagnosis
Cases may be identified from a single or multiple
sources
Generally report on new/unique condition
A realistic design for rare disorders .
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Advantages
Hypothesis triggering
Informative for very rare disease with few
established risk factors
Disadvantages
Cannot study cause and effect relationships
Cannot assess disease frequency
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CROSS SECTIONAL STUDIES
An “Observational” design that studies exposures
and disease status at a single point in time (a
cross-section of the population)
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CROSS-SECTIONAL DESIGN
time
Study only exists at this point in time
Study
population
No Disease
Disease
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It measures prevalence, not incidence of disease
Example: community surveys
Not suitable for studying rare or a disease with
short duration of expression
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PROCEDURES
1.Defining the population to be studied
2.Defining the disease under study
3. Measurement of disease
4. Comparing with known indices
5.Formulation of an aetiological hypothesis
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STEP 1: DEFINING THE
POPULATION
A defined population should not only be in terms of
total number
but also in terms of age, sex, location etc.
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The defined population-
i) could be a whole geographic region or;
a representative sample not be different from
other
communities in the region
ii) large enough
iii) stable
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“SNAPSHOT STUDY”
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STEP 2: DEFINING THE
DISEASE
 define the disease which can be
• measured and
• identified with a degree of accuracy.
 could be different from the clinician’s definition
of a disease
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STEP 3:MEASUREMENT OF DISEASE
Data collection
• Clinical examination
• Questionnaires
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STEP 4: COMPARING WITH KNOWN
INDICES
Comparisons are made with known indices to
arrive at clues to the disease’s etiology
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Cross sectional
Descriptive Analytical
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Prevalence Analysis to find out whether - any
association with a characteristic
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STEP 5: FORMULATION OF A
HYPOTHESIS
A hypothesis is a supposition, arrived at from
observation or reflection.
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USES OF DESCRIPTIVE EPIDEMIOLOGY
 data regarding the magnitude of the disease load
and types of disease problems.
 clue to disease etiology and help in the
formulation of an etiological hypothesis.
 background data for planning, organizing and
evaluating preventive service.
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ANALYTICAL
EPIDEMIOLOGY
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ANALYTICALEPIDEMIOLOGY.
Is a type of observational
study
Test the hypothesis.
Investigator measures
but does not intervene
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BASIC QUESTION IN ANALYTIC
EPIDEMIOLOGY
 Are exposure and disease linked?
Exposure Disease/
Outcome
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 Retrospective study
 Unit of study: Cases/ Control ( Individuals)
 Direction of Inquiry : E  O
 Match controls to cases based on major factors
related to the disease.
CASE-CONTROL DESIGN
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CASE-CONTROLDESIGN
Study
population
Dental
erosion
(disease)
No dental
erosion
(no disease)
Alcoholism
present
Alcoholism
absent
Alcoholism
present
Alcoholism
absent
present
past
time
Study begins here
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1. Selection of Cases and Controls
Selection Sources of
cases
Hospitals
General
Population
Diagnostic
Eligibility-
(Operational
definition-
ADA)
Sources of
controls
Hospitals
General Population
Relatives
Neighbourhood 91
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II. Matching
 Defined as the process by which we select
controls in such a way that they are similar to
cases with regards to certain pertinent selected
variables (e g. age, sex, occupation, social status
etc. ) which are known to influence the outcome of
the disease.
Cases
Controls
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III. Measurement of Exposure
Information
- interviews
- questionnaires
-studying past hospital records,
employment records etc.
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IV. Analysis
- Exposure rate
-Estimation of disease risk associated with
exposure(odds ratio)
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Exposure rates
 Direct estimation of the exposure rates to a
suspected factor in diseased and non – diseased
groups.
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EXPOSURE OF CASES = A /(A+C) =
80/100 = 80 %
EXPOSURE OF CONTROLS = B / (B+D) =
40/100 = 40 %
Cases
(Dental erosion)
Controls (without
Dental erosion)
Alcohol users 80 (a) 40 (b)
Non- Alcohol
users 20 (c) 60 (d)
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Odds Ratio (Cross Product Ratio)
The OR represents the odds that an outcome will
occur given a particular exposure, compared to
the odds of the outcome occurring in the absence
of that exposure.
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Cases
(Dental
erosion)
Controls
(without
Dental
erosion)
Alcohol
users
80 (a) 40 (b)
Non-
Alcohol
users
20 (c) 60 (d)
ad 80 x 60
Odds ratio = bc 40 x 20
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Cases
(Dental erosion)
Controls (without
Dental erosion)
Non- Alcohol users
20 (c) 60 (d)
ad 80 x 60
Odds ratio = = = 6
bc 40 x 20
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Beaglehole R, Bonita R, Kjellström T. Basic Epidemiology. 2nd ed. Geneva:
World Health Organization; 2006.

CASE-CONTROL STUDY
Strengths
Less expensive and time consuming efficient for
studying rare diseases.
Limitations
Exposure measurements taken after disease
occurrence
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Cohort Study
 Longitudinal study, Incidence study,
Forward-looking study.
• Best observational design
• Looking for a difference in the risk
(incidence) of a disease over time
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COHORT STUDIES
Cohort study is a type of analytical study which is
undertaken to obtain additional evidence to refute or
support existence of association between suspected
cause and diseases.
Other names of cohort study are Longitudinal study,
Incidence study and forward looking study.

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Features of cohort studies
• Cohorts are identified prior to appearance of
disease under investigation.
• The study groups are observed over a period of
time to determine the frequency of disease among
them.
• The study proceeds from cause to effects.

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Indications for cohort study
• There is good evidence of an association between
exposure and disease, from other studies.
• Exposure is rare.
• Sufficient fund is available.

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Consideration during selection of Cohort
• The cohort must be free from disease under study.
• Both the groups must be comparable in respect of
all variable which influence the occurrence of
disease.
• Diagnostic and eligibility criteria of the disease
must
be defined beforehand.

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Types of cohort study
• Prospective study
• Retrospective cohort study
• Ambi-directional cohort study

PROSPECTIVE COHORT STUDY
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In the prospective cohorts study, by the time your
study starts exposure and disease has not yet
occurred.

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Retrospective Cohort Study
• A retrospective cohort study is one in which the
outcome have all occurred before the start of
investigation.
• Investigator goes back to the past to select study
group from existing records of the past
employment, medical and other records and traces
them forward through time from the past date fixed
on the records usually to the present.
• Known with the name of Historical Cohort and
noncurrent cohort

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THE ROLE OF ANILINE DYES AND
DEVELOPMENT OF URINARY BLADDER
CANCER. The investigator recruited about 4622
workers who were working in dyes industry
between 1920 and 1951. So, this recruitment was
based on available records in those factories.
Investigator also revived the death records of this
4622 individuals and looked about any mention of
urinary bladder tumors on their death records, and
then they compared death rates in these
population with that of expected number of deaths
of bladder cancer using national statistics. So, by
the time the studies started, both exposure and
outcome had occurred.

AMBISPECTIVE STUDY
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A combination of these two approaches, which is
called as Bidirectional study or Ambispective study,
wherein when your study starts, the exposure has
already occurred and then you follow this exposed
and then exposed individuals, till they develop the
outcome.

 Relative Risk

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(a/a+b)
Relative risk= = 2.7
(c/c+d)
Cases
(Dental erosion)
Controls (without
Dental erosion)
Non- Alcohol
users 20 (c) 60 (d)
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COHORT STUDY
Strengths
 Exposure status determined before disease
detection
 Subjects selected before disease detection
 Can study several outcomes for each exposure
Limitations
 Expensive and time-consuming
 Inefficient for rare diseases or diseases with
long latency
 Loss to follow-up
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EXPERIMENTAL STUDY
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•Experimental or intervention studies are similar
in approach to cohort studies excepting that the
conditions in which study is carried out are
under the direct control of the investigator
EXPERIMENTAL STUDIES:
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Thus experimental studies involve some action,
intervention or manipulation such as deliberate
application or withdrawal of the suspected cause or
changing one variable in the causative chain in the
experimental group while making no change in the
control group, and observing and comparing the
outcome of the experiment in both the groups.
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AIMS
1. To provide “scientific proof” of aetiological or risk
factors which may permit the modification or
control of those diseases.
2. To provide a method of measuring the
effectiveness and efficiency of health services for
the prevention, control and treatment of diseases
and improve the health of the community.
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THE EXPERIMENTAL DESIGN
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Reference/target population
Experimental/study population
Participant population Non-participant population
Intervention
group
Control
group
outcome outcome
compare
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RANDOMIZED CONTROLLED TRIALS
• IT IS REALLY AN EPIDEMIOLOGIC
EXPERIMENT.
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 Definition: (Stedman’s medical dictionary)
 RCTs are quantitative, comparative, controlled
experiments in which investigators study two or
more interventions in a series of individuals who
receive them in random order.
 The RCT is one of the simplest and most powerful
tools in clinical research.
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BASIC STEPS IN CONDUCTING A RCT INCLUDE:
1. Drawing up a protocol
2. Selecting reference and experimental population
3. Randomization
4. Manipulation or Intervention
5. Follow-up
6. Assessment of outcome
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1. DRAWING UP A PROTOCOL
 Strict protocol
 Aims & objectives
 Questions to be answered
 Criteria of selection: Study & control groups
 Intervention to be applied
 Standardization of working procedures
 Schedules
 Strictly followed through the study
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Preliminary test runs:
• Sometimes, before a protocol is completed,
preliminary(pilot) studies have to be made to find
out the feasibility or operational efficiency of
certain procedures or unknown effects or the
acceptabililty of certain policies.
• Sometimes it is useful to have a short test run of
the protocol to see whether it contains flaws.
• It is important that the final version of the protocol
should be agreed upon by all concerned before
the trial begins.
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2. SELECTING REFERENCE AND EXPERIMENTAL POPULATION:
a) Reference or target population:
 It is the population to which the findings of the
trial if found successful, are expected to be
applicable.
 It may be as broad as mankind or it may be
geographically limited or limited to persons in
specific age, gender, occupational or social
groups.
 Eg: population of the whole city, school children,
industrial workers etc
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Experimental /Study Population:
 It is derived from the reference population.
 randomly chosen from the reference population.
 same characteristics as reference population.
 choose a stable population whose co-operation is
assured to avoid losses to follow-up.
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The participants should fulfill the following criteria:
1. They must give informed consent.
2. Should be representative of the population.
3. Should be qualified or eligible for the trial.
Eg: new drug : treatment of anaemia.
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3. RANDOMIZATION
 “Heart" of a control trial.
 It is the statistical procedure by which the
participants are randomly allocated into groups
usually called the "study" and "control" groups.
 It is an attempt to eliminate "bias" and allow for
comparability.
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• Randomization eliminates “selection bias”
• Randomization is done only after the participant
has entered the study and it can be done by
using table of random numbers.
• In otherwords, by random allocation, every
individual gets an equal chance of being allocated
into either group or any of the trial groups.
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 The essential difference between a randomized
controlled trial and an analytical study is that the
latter, there is no randomization because a
differentiation into disease and non-disease
(exposed and non-exposed) groups has already
taken place.
 The only option left to ensure comparability in
analytical studies is by matching.
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4. MANIPULATION
• This refers to the deliberate application or
withdrawal of the suspected causal factor as laid
down in the protocol.
• Eg : drug, vaccine, dietary component, habit
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Manipulation creates
• An independent variable (drug, vaccine, new
procedure) whose effect is then determined by
the measurement of the final outcome, which
constitutes the dependant variable.(incidence of
disease, survival time, recovery period)
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 5. FOLLOW-UP
 Examination of groups at defined intervals of time
under the same conditions, in a standard manner,
with equal intensity, same circumstances, same
time frame.
 Follow-up can vary from a short period to many
years.
 Over the years, there may be loss of subjects
from either group due to a number of reasons.
This is called as “attrition”.
 Death
 Migration
 Loss of interest
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6. ASSESSMENT
The final step of the outcome of he trial in terms of:
a) Positive results: that is, benefits of the
experimental measure such as reduced
incidence or severity of the disease, cost to the
health service or other appropriate outcome in
the study and control groups.
b) Negative results: that is, adverse effects, severity
and frequency of side effects and complications,
if any including death.
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 Errors in assessment can lead to “Bias”.
 Bias can arise from three sources:
1. Subject variation
2. Observer bias
3. Evaluation Bias
 Randomization cannot guard against
these sort of bias.
To avoid the above situations, “Blinding”
is done.
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WHAT IS BIAS??
Bias is any systematic error in the determination of
the association between the exposure and disease.
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TYPES OF BIAS:
1. Non-representative sample drawn from study
population:
- Random sampling
2. Biased allocation of subjects done to the
intervention and control group:
- Ensure ‘random allocation’
3. Observers bias:
- Ensure double blinding
4. Respondents bias:
- Ensure atleast single blinding or placebo for
controls.
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5. Contamination bias:
( due to co-intervention with some other
intervention in addition to the intervention under
study, by the subjects)
- Explain in detail the requirement of your
research to the subjects and win over their
cooperation; keep evaluating for any possible co-
intervention by subjects.
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6. Bias due to dropouts and loss of follow up:
- Ensure periodic follow up
- develop methods to retrieve the lost subjects.
- Compare the characteristics of those who
remain in study with those have been lost to
follow-up and see if there are significant
differences between them.
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10. Confounding bias:
- Ensure random allocation to the groups.
- do a baseline comparision.
- record the data on confounders for
subsequent
treatment during analysis.
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BLINDING
 In experiments, more specifically in clinical trials it
is a researcher knows which treatment is given to
which subjects, there is likelihood of bias
creeping in while assessment of response by
investigator.
 Bias can also creep in while describing response
to investigator in case participating subjects know
the treatment given.
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 Single blind: Participant
 Double blind : Participant + Investigator
 Triple blind : Participant + Investigator + Analyzer
Blinding is not so important when the final
outcome will be death of the patient.
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Methods for achieving double blinding
MATCHING
If two treatments are available in the form of tablets
then matching both tablets in terms of color, shape,
strength, texture etc.
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STUDY DESIGNS OF CONTROLLED TRIALS
1. CONCURRENT PARALLEL STUDY DESIGN
2. CROSS-OVER TYPE OF STUDY DESIGNS
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TYPES OF RANDOMIZED CONTROLLED TRIALS
1. Clinical trials
2. Community intervention trials
3. Preventive trials
4. Risk factor trials
5. Cessation experiment
6. Trial of aetiological agents
7. Evaluation of health services
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CONCLUSION
EPIDEMIOLOGY IN PUBLIC HEALTH
1. Preventing disease and promoting health.
2. Community health assessment (Community
Diagnosis) and priority setting.
3.Improving diagnosis, treatment and prognosis
of clinical diseases.
4.Evaluating health interventions and programmes
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BIBLIOGRAPHY
1.Park's Textbook of Preventive and Social
Medicine-24TH EDITION.
2.Jekel's Epidemiology, Biostatistics, Preventive
Medicine, and Public Health, 4th Edition.
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154
3.Epidemiology. Leon Gordis 3rd Edition.
4.Basic epidemiology. R.Bonita, R.Beaglehole,
T. Kjellstrom 2nd Edition

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