2. 2
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.
3. 3
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)
5. 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.
9. 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.).
12. 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.
12
13. 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.)
14. Ex: The Percentage of under five children
with acute malnutrition in Africa in March
2008
15. 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.)
15
16. 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.
18. 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.
19. 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%
20. 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
20
21. 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.
22. 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.
23. 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
24. 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)
25. 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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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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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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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”
36. 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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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…
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
