Fundamentals and Study Design of Epidemiology

Fundamentals of Epidemiology,
Epidemiologic Methods, Design and
Conduct of Clinical Trials
Faraza Javed
Ph.D Pharmacology

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Introduction
The term epidemiology is derived from the Greek word
epidemic.
Epi means-Among, upon,
Demos means study population or people and
Logos means scientific study.
So
It is the scientific study of the disease pattern in human
population.
In broad sense, it is the study of effects of multiple factors
on human health.
It is multidisciplinary subject involving those of the
physician, Biologists, Public Health experts, Health
educators etc.

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The science of the mass phenomena of infectious
diseases or the natural history of infectious diseases.
(Frost 1927)
The science of infective diseases, their prime causes,
propagation and prevention. (Stallbrass 1931)

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The widely accepted definition of epidemiology is:
“The study of the distribution and determinants of
health related states or events in specified
population and the application of the study to
control of health problems”
(J.M. Last 1988)

Aims of Epidemiology
According to the International Epidemiological Association
(IEA) Epidemiology has three main aims.
To describe and analyze diseases occurrence and
distribution in human populations;
To identify etiological factors in the pathogenesis of
diseases;
To provide the data essential to the planning,
implementation and evaluation of services for the
prevention, control and treatment of diseases and to the
setting up of priorities among those services.

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Components of Epidemiology
Disease frequency:
 The core characteristics of epidemiology are to
measure the frequency of diseases, disability or death
in a specified population. it is always as the rate, ratio
and proportion.
 Much of the subject of epidemiology are matter of
measurement of diseases and health related events
falls in the domain of biostatistics, which is a basic
tool of epidemiology. This helps in development of
strategies for prevention or control of health related
problems.

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Distribution of Diseases:
 Health events occur in pattern in community and
this pattern varies from community to community.
 Also health events or diseases condition affect
population at various age groups, different genders,
different subgroups of population.
 Distributions of events are based on time, place, and
person. We can analyze whether any increases or
decreases occur for a particular condition.
Epidemiology addresses itself to a study of these
variations or patterns, which may suggest or lead to
measure to control or prevent the diseases. An
important outcome of this study is formulation of
etiological hypothesis.

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Determinants of Diseases:
 Epidemiology helps in identifying the causative
agent or the risk/predisposing factors of diseases.
 This is one of the real uses of epidemiology.
Understanding the factors leading to any programs
for the control of those diseases.

How to study the disease in the
community?
Clinical Method: Studied subject is a patient
(individual person) and decision on his/her
treatment requires a clinical diagnosis (based
on the history, examination, laboratory tests,
etc.).

Epidemiological Method: Epidemiology
studies not only an individual, but also a
whole population. Community diagnosis is
essential and can be expressed in terms
of rates.

Basic Measurement
 Prevalence Rate
 Incidence rate
 Case fatality rate
 Mortality rates(age specific/cause specific)
 Attack rate
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Prevalence vs. Incidence
Prevalence: frequency of existing cases
Incidence: frequency of new cases
New cases are called incident cases.
Existing cases are called prevalent cases.
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PREVALENCE RATE
No. of people with disease at specified time/No. of
people in Population at risk at specified time
x 1000
Prevalence rate is often used to measure the occurrence of
chronic (long lasting) diseases or diseases with gradual
onset (such as diabetes, sclerosis multiplex, tuberculosis
etc.)

Ex: The Percentage of under five children
with acute malnutrition in Africa in March
2008

INCIDENCE RATE
Incidence Rate is defined as the no. of NEW cases occurring
in a defined population during a specified time period.
No. of new cases of specific disease during a given time
period/ Population at risk during that period
X 1000
Incidence rate is often used to measure the occurrence of
acute (short-term) diseases or diseases with exactly defined
onset (such as acute intestinal diseases, poisonings, car
accidents, strokes, etc.)
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For example , if there had been 500 new cases of
an illness in a population of 30,000 in a year, the
incidence rate would be :
500/30000 x 1000 = 16.7
 Incidence rate refers to only new cases.

ATTACK RATE
The attack rate, or case rate, refers to the
cumulative incidence of infection over a period
of time. This is typically used during an
epidemic. The time period may not be indicated,
but would typically refer to the period of the
outbreak.

Ex:
Outbreak of cholera in country X in March 1999
Number of cases = 490,
Population at risk = 18,600
Then the Attack rate = 2.6%

CASE FATALITY RATE
Measure of the severity of a disease which defined as the
proportion of cases of a specified disease or condition
which are fatal within a specified time
= no. of death from a disease in a specified period
no. of diagnosed cases of disease in same period
X 100
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The proportion of people with a specified condition who
die within a specified time. The time frame is typically
the period during which the patient is sick from the
disease. This works for an infectious disease but can be
problematic for a chronic disease like a cancer that may
remit for a period and then prove fatal after a
recurrence. In such instances we tend to speak of
mortality or survival rates rather than case fatality.

MORTALITY RATE
The number of deaths per thousand population per
year: in effect, the incidence of death in a
population. It can refer to all causes of death, or
can be a cause-specific mortality rate.

It expresses the no. of death due to a particular cause (or in
a specific age group) per 100 (or 1000) total deaths
No. of deaths from the specific disease in a year x 100
Total deaths from all causes in that year

EPIDEMIOLOGICAL STUDIES
When we need to obtain more precise or detailed data, it
may be necessary to undertake a special survey.
Planning of the study:
Definition of the aim of the study and type of the
study.
Determination of the studied population (target
population)-to exactly define the attributes of
individuals belonging to this population (including
and excluding criteria)

 Case definition – to define a tool for discrimination
positive and negative cases. This tool (method)
should be cheap, simple and readily available.

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Epidemiologic Study Designs

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Study Design
Study design is the arrangement of conditions for the
collection and analysis of data to provide the most accurate
answer to a question in the most economical way.

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Types
1. Cross-sectional studies
2. Case-control studies
3. Cohort studies
4. Experimental studies

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Cross-sectional Studies
 In this study design information about the status of an
individual with respect to presence/absence of exposure
and diseased is assessed at a point in time.
 Cross-sectional studies are useful to generate a hypothesis
rather that to test it
 For factors that remain unaltered overtime (e.g. gender,
race, blood group) it can produce a valid association

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Cross-Sectional…
 Comparison groups are formed after data collection
 The object of comparison are prevalence of exposure or
disease
 Groups are compared either by exposure or disease status
 Cross-sectional studies are also called prevalence studies

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Cross-sectional…
Types of cross-sectional studies
1. Single cross-sectional studies
Determine single proportion/mean in a single
population at a time
2. Comparative cross-sectional studies
Determine two proportions/means in two
populations at a time

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Cross-sectional…
Advantages of cross-sectional studies
 Less time consuming
 Less expensive
 Provides more information
 Describes well
 Generates hypothesis

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Cross-sectional…
Limitations of cross-sectional studies
 Antecedent-consequence uncertainty
“Chicken or egg dilemma”
 Data dredging leading to inappropriate comparison
 More vulnerable to bias

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Case-Control Studies
 Subjects are selected with respect to the presence (cases)
or absence (controls) of disease, and then inquiries are
made about past exposure
 We compare diseased (cases) and non-diseased (controls)
to find out the level of exposure
 Exposure status is traced backward in time

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Case-control…
Steps in conducting case-control studies:
Define who is a case
Establish strict diagnostic criteria
All who fulfil the criteria will be “case population
Those who don’t fulfil will be “control population”

Individuals are divided according to presence of the
disease: studied group (disease present, e.g. lung
cancer) and control group (disease absent, e.g.
without lung cancer). Occurrence of suspected risk
factor in history is compared in both groups (e.g.
proportion of smokers) (studies focused from
consequence to cause)

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Case-control…
Advantages of case-control studies
 Optimal for evaluation of rare diseases
 Examines multiple factors of a single disease
 Quick and inexpensive
 Relatively simple to carry out
 Guarantee the number of people with disease

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Cohort studies
Subjects are selected by exposure and followed to see
development of disease
Two types of cohort studies
1. Prospective (classical)
 Outcome hasn’t occurred at the beginning of the
study
 It is the commonest and more reliable

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Cohort…
2. Retrospective (Historical)
 Both exposure and disease has occurred before the
beginning of the study
 Faster and more economical
 Data usually incomplete and in accurate

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Cohort…
Steps in conducting cohort studies
 Define exposure
 Select exposed group
 Select non-exposed group
 Follow and collect data on outcome
 Compare outcome b/w exposed & non-exposed

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Cohort…
Advantages of cohort studies
 Valuable when exposure is rare
 Examines multiple effects of a single exposures
 Allow direct measurement of risk

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Cohort…
Limitations of cohort studies
 Expensive
 Time-consuming
 Loss to follow-up creates a problem

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Experimental studies
Individuals are allocated in to treatment and control
groups by the investigator. If properly done,
experimental studies can produce high quality data.
They are the gold standard study design.
They included studied group (intervention, e.g.
vaccinated children), control group (without
intervention, e.g. non-vaccinated children)

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Experimental…
The quality of “Gold standard” in experimental
studies can be achieved through
Randomization
Blinding
Placebo

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Experimental…
Randomization: random allocation of study subjects in to
treatment & control groups
Advantage: Avoids bias & confounding
Increases confidence on results

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Experimental…
Single blinding: Study subjects don’t know to which
group they belong
Double blinding: Physicians also don’t know to which
group study subjects belong
Triple blinding: Data collectors also don’t know
allocation status
Advantage: Avoids observation bias

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Experimental…
Placebo: An inert material indistinguishable from active
treatment
Placebo effect: Tendency to report favourable response
regardless of physiological efficacy
Placebo is used as blinding procedure

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Screening
Screening refers to the presumptive identification of a
disease/defect by application of tests, examinations or
other procedures in apparently healthy people.
 Screening is an initial examination
 Screening is not intended to be diagnostic

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Aims of screening program
 Changing disease progression efficiently
 Altering natural course of disease
 Protecting society from contagious disease
 Allocating resources rationally
 Selection of healthy people for job
 Studying the natural history of disease

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Criteria for establishing screening
program
 The problem should have public health importance
 There should be accepted treatment for positives
 Diagnostic & treatment facilities should be available
 Recognized latent stage in the time course
 Test is acceptable, reliable & valid
 Natural history of the disease should be well
understood
 Case-finding is economical and continuous

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In general, a screening test should be
Reliable & valid
Sensitive & specific
Simple & acceptable
Effective & efficient

Fundamentals and Study Design of Epidemiology

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Fundamentals and Study Design of Epidemiology