INTRODUCTION TO EPIDEMIOLOGY

COMMUNITY HEALTH NURSING-I
B.Sc(N) II Year
UNIT – III
EPIDEMIOLOGY
S.NIRANJANI
ASSOCIATE PROFESSOR
TAGORE COLLEGE OF NURSING

DEFINITION
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

AIMS OF EPIDEMIOLOGY
According to the international epidemiology association,
epidemiology has 3 main aims
i. To describe the distribution and magnitude of health and
disease problems in human population.
ii. To identify etiological factors in the pathogenesis of disease
iii. 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.

The ultimate aim of epidemiology is to lead
to effective action:
a. to eliminate or reduce the health problem or
its consequences
b. To promote the health and well – being of
society as a whole.

USES OF EPIDEMIOLOGY
i. To study historically the rise and fall of disease in the
population.
ii. Community diagnosis
iii. Planning and evaluation
iv. Evaluation of individual’s risk and chances
v. Syndrome identification
vi. Completing the natural history of disease
vii. Searching for causes and risk fcators.

SCOPE OF EPIDEMIOLOGY
According to disease group;
i. Infectious disease epidemiology
ii. Cardio vascular epidemiology
iii. Cancer epidemiology
iv. Neuro epidemiology
v. Nutritive epidemiology
vi. Reproductive epidemiology
vii. Environmental epidemiology

viii. Clinical epidemiology
ix. Molecular epidemiology
x. Genetic epidemiology
xi. Computational epidemiology
xii. Micro epidemiology
xiii. Comparative epidemiology
xiv. Macro epidemiology

HUMAN RESERVOIR
• By far the most important source or reservoir of
infection for humans is man himself
• He may be case or carrier
CASE:
A case is defined as a person in the population or
study group identifies as having the particular disease,
health disorder or condition under investigation.
Broadly the presence of infection in a host may be
clinical, sub clinical or latent

i. The clinical cases:
epidemiologically, mild cases may be more
important sources of infection than severe cases
because they are ambulant and spread the infection
wherever they go.
ii. The sub clinical cases:
The disease agent may multiple in the host
but doesn't manifest itself by signs and symptoms.
sub clinical infection can be detected only by
laboratory tests.

iii. The latent infection:
In latent infection, the host
doesn't shed the infectious agent which lies
dormant within the host without symptoms.

CARRIERS
• In some diseases, either due to inadequate
treatment or immune response the disease agent is
not completely eliminated, leading to a carrier
state.
• A carrier is defined as an infected person or
animal that harbours a specific infectious agent in
the absence of discernible clinical disease and
serves as a potential source of infection for others.

• As a rule carriers are less infectious than cases
but epidemiologically, they are more
dangerous than cases because they escape
recognition and continuing as they do to live a
normal life among the population or
community

TYPE:
i. Incubatory carrier:
Incubatory carriers are those who
shed the infectious agent during the incubation
period of disease eg. Measles, mumps.
ii. Convalescent carriers:
those who continue to shed the
disease agent during the period of
convalescence. Eg. Typhoid fever, cholera

iii. Healthy carrier:
Healthy carrier emerge from sub
clinical cases.
They are victims of sub clinical
infection who have developed carrier state
without suffering from overt disease, but are
nevertheless shedding the disease agent eg.
Polio, cholera.

By duration:
i. Temporary carriers:
Temporary carriers are those who shed the
infectious agent for short periods of time.
ii. Chronic carriers:
A chronic carrier is one who excretes the
infectious agent for indefinite periods
Eg. hepatitis B
iii. By portal of exit:
Thus we have urinary carriers, intestinal carriers
and respiratory carriers.

Animal reservior
• The source of infection may sometimes be
animals and birds.
• The disease and infections which are
transmissable to man from vertebrates are
called zoonoses
• Eg rabies, yellow fever

Reservoir in non living things
• Soil and inanimate matter can also act as
reservoirs of infection
Eg. Tetanus, anthrax.

MODES OF TRANSMISSION
• Communicable disease may be transmitted from the
reservoir or source of infection to a susceptible
individual in many different ways, depending upon the
infectious agent, portal of entry and the local ecological
conditions.
• As a rule an infectious disease is transmitted by only
one route, but there are others which may be
transmitted by several routes
Eg. AIDS, Hep B

DIRECT TRANSMISSION
i. Direct contact: direct contact from skin to skin , mucosa to
mucosa
ii. Droplet: droplets of saliva and naso pharyngeal secretions.
iii. Contact with soil: tetanus
iv. Inoculation into skin: inoculated directly into the skin and
mucosa
v. Transplacental: TORCH( toxoplasma gondii, rubella virus,
cytomegalo virus and herpes virus)

INDIRECT TRANSMISSION
i. Vehicle borne: water, food, blood
ii. Vector borne: arthropods
iii. Airborne
a. Droplet nuclei: tiny particles (1-10 microns)
b. Dust: larger droplets settle down on the flooe,
carpets, furnitures.
iv. Fomit borne: cloths, towels, linen etc..
v. Unclean hands and fingers:

SUSCEPTIBLE HOST
• The infectious agent enters the susceptible host after
finding the portal of entry such as respiratory tract,
alimentary canal etc..
• Inside the human host, on getting appropriate
environment, it multiply and sufficient density of
the disease agent is build up to disturb the health
equilibrium and the disease become overt.

SUSCEPTIBLE HOST
Disease which occur in human host goes through the
various stages;
i. Incubation period
ii. Prodromal period
iii. Fastigum
iv. Defervescence
v. Convalescence
vi. defection

MECHANICAL AGENT:
Mechanical forces such as friction result in
crushing, tearing, dislocation and sometimes death.
SOCIALAGENT:
social problems like poverty, smoking,
alcohol and social isolation
NUTRIENT AGENT:
An excess or deficiency of intake can
result in nutritional deficiency disorder eg. goitre,
anaemia,rickets.

HOST
Host factor is the human body
in which the disease agent stays, grows and
multiply to produce illness

HOST
FACTOR
Biological
characteristics
Genetic factor
Life style
Living habits
Physical
exercise
Nutrition, use
of alcohol and
smoking
Socio economic
characteristics
Education,
occupation,marital
status
Demographic
characteristic
Age, sex and
ethnicity

ENVIRONMENTAL FACTOR
Environment
Internal External
Internal factors of Physical envt biological psycho sociol
human body

i. Physical environment:
Non living things and the physical
factors such as air, water, housing, heat, light.
ii. Biological environment:
All living things such as microbial
agents, insects, rodents.
iii. Psychosocial environment:
cultural values, habits, customs, beliefs,
lifestyle etc..

MEASUREMENT OF MORBIDITY AND
MORTALITY
Scope of measurement in epidemiology is
broad and unlimited and includes the
measurements of morbidity, disability, natality
etc..
Epidemiologist usually expresses the disease
rate, ratio and proportion.

RATE
 A rate is the occurrence of some particular event in a population
during a given time period.
 It comprises a numerator, denominator, multiplier and time
specifications.
 The rate is expressed per 10000, 1000 or 100 according to
convenience to avoid fractions.
 Various categories of rates are
i. Crude rate
ii. Specific rate
iii. Standardized rates.

RATIO
 Another measure of disease frequency is a ratio
 It expresses a relation in size between two random quantities
 The numerator is not a component of the denominator
 Broadly ratio is the result of dividing one quantity by another, it is
expressed in the form of x:y or x/y.
For eg.
1. The number of children with scabies at a certain time
The number of children with malnutrition at a certain time.
2. The ratio of WBC relative to RBC is 1:600 , meaning that for each
WBC, there are 600 red cells.

PROPORTION
 A proportion is a ratio which indicates the relation in
magnitude of a part of the whole
 The numerator is always includes in the denominator
 A proportion is usually expressed as a percentage
 Eg.. The number of children with scabies at a
certain time x 100
The total number of children in the village
at the same time

MORTALITY RATES AND RATIOS
i. Crude death rate:
It is defined as the number of deaths
from all causes per 1000 estimated mid year
population in 1 year in a given place.
Number of deaths during the year x 1000
Mid year population

Specific death rate:
 when analysis is planned to throw light on etiology,
it is essential to use specific death rates.
 The specific death rates may be cause or disease
specific e.g.. TB, cancer
 Related to specific group eg. Age , sex- specific

a. Specific death rate = Number of deaths from
due to tuberculosis TB during a calendar year x 1000
Mid year population
b. Specific death rate in = number of deaths of persons aged
age group 15 -20 years 15 – 20 during calendar year x 1000
mid year population of persons
aged 15 - 20

Case fatality rate:
CFR = Total number of deaths due to
particular disease
Total number of case due to the x 100
same disease
• CFR represents the killing power of a disease
• CFR is typically used in acute diseases, its usefulness
for chronic disease is limited

Survival rate:
• It is a method of describing prognosis in
certain disease conditions
• Survival rates have received special attention
in cancer studies
Survival rate = Total number of patients alive
after 5 years x 100
Total number of patients
diagnosed or treated

Standardized mortality ratio:
• If we want to compare the death rates of two
populations with different age – composition, the
crude death rate is not the right yardstick.
• Standard mortality ratio is a ratio of the total
number of deaths that occur in the study group to
the number of deaths that would have been
expected to occur of the study group, had
experienced the death rates of a standard
population

SMR = observed death
expected deaths x 100

MEASUREMENT OF MORBIDITY
• Morbidity means deviation from normal state
of physiological well- being.
• Morbidity is measured in terms of
i. Frequency
ii. Duration
iii. Severity.

1. Disease frequency:
It is measured by incidence rate
and prevalence rate.
a. Incidence rate:

• Prevalence rate is calculated either at one point
of time or defined period of time.
• Depending upon this it is of two types
i. Point prevalence:
number of current new and old cases
of a specified disease existing at a given
point of time
x 100
estimated population at the same
point of time

Period prevalence:
number of current new and old cases
of a specified disease during a given
period of time interval
x 100
estimated mid – interval population at risk

DISABILITY RATE
Indicators used to measure disability
i. Sullivan index: it is calculated as given below
Life
Expectancy - Probable duration of bed disability + inability to
perform major activities

ii. Health adjusted life expectancy: ( HALE)
 It is an indicator which measures healthy life
expectancy.
 Earlier disability adjusted life expectancy ( DALE)
was measured which has been changed to HALE.
 It is equivalent to the number of years newborn
child expected to live with full health.

iii. Disability adjusted life year:
It is used to express the years of life lost to
premature death and years lived with disability for
severity of disability.
One DALY = one lost year of healthy life.

PRIMORDIAL PREVENTION
 It refers to the prevention of appearance or
development of risk factors in countries or population
groups in which they have never appeared before.
 Primordial prevention, efforts are intended for
discouraging children from adopting hazardous
lifestyles
 The main intervention in primordial prevention is
through individual and mass education.

PRIMARY PREVENTION
 A primary prevention goal relies upon holistic approach
that signifies intervention in the pre-pathogenesis
phase.
 In primary prevention, necessary preventive measures
are taken prior to the onset of the disease, which
removes the likelihood of a disease occurrence.
 It promotes individual’s attitude towards life and health
and the necessary steps he takes for himself, his family
and community.

 Primary prevention includes
i. Health promotion and
ii. Specific protection.
i. Health promotion:
 Health promotion is considered to be a behavioral social
science that comes from the biological, environmental,
psychological, physical, medical sciences to promote
health and prevent disease, disability and pre mature
death through educational and voluntary behavioural
change activities.

Health promotion approaches are
i. Health education
ii. Environmental modification
iii. Nutritional interventions
iv. Lifestyle and behavioral changes.
i. Health education:
health education is defined as consciously constructed
opportunities for learning involving some form of
communication designed to improve health literacy,
including improving knowledge and developing life skills,
which are conducive to individual and community health.

ii. Environmental modification:
 There is a mutual relationship subsists between human health
and the environment that is known from many evidences.
 Environmental modification are thought to be an important
addition to health promotion
 Poorly ventilated fire places, stoves, water quality, food
sources and food quality contribute to ill health.
 Environmental protection agency reports on the relationship
between long term use of pesticides and birth defects ,
nerves damage and other long term developmental problems
in children.

Man cannot separated from his environment.
All possible measures should be taken to
promote healthy environment
All potential man made disaster and pollution
activities should be avoided or controlled to
keep our environment safe for living.

iii. Nutritional modifications:
 Malnutrition impairs the growth and development and causes death
of many million children.
 In general, healthy nutrition contributes to the physiological, mental
and social well being of individuals.
 Nutrition interventions are designed to promote health and reduce
the risk of diseases
 Adequately nourished people enjoy optimal growth, health and well
being.
 India has launched many nutritional programs sponsored by central
or state governments in order to promote the health of children,
antenatal and postnatal women as well as old age people.

iv. Lifestyle modification:
 New research conducted in Iran shows that lifestyle changes in diet
and levels of physical activity improve the health of entire
communities.
 The lifestyle problems are smoking, drinking alcohol, using drugs,
engaging in unsafe sex or being over weight or obese.
 The biggest contribution to most preventable diseases are;the
economic status, tobacco, alcohol, physical inactivity and poor diet
 These four lifestyle behaviors need to be targeted to improve the
health of the nation.

2. specific protection.
 Specific protection activities focuses on enhancing resistance to some
factors like viruses and bacteria or modifying the environment to decrease
potentially harmful interactions such as exposure to toxic agents in the
environment.
 Some examples for specific protection are
i. Immunisation
ii. Use of specific nutrients
iii. Chemoprophylaxis
iv. Protection against occupational hazards
v. Protection against accidents and carcinogens
vi. Avoidance of allergens
vii. Control of hazards in the environment like air pollution, noise pollution
etc..

SECONDARY PREVENTION
Early diagnosis and treatment:
i. It helps in detecting a disease in its earliest stage, before symptoms appear
and intervening to slower stop its progression; “catch it early”.
ii. The assumption is that earlier intervention will be more effective and that
the disease can be slowed or reversed.
iii. Secondary prevention measures are delivered in hospital, homes and other
secondary facilities.
iv. It comprises of diagnostic screening programs, public education to
promote self examinations to be able to recognize early signs and
symptoms of certain diseases.
v. Screening tests or specific procedures are used to detect serious diseases
as early as possible so that the progress can be detained eg.pap test to
screen ca cervix.

TERTIARY PREVENTION
 Tertiary prevention level begins early in the period of the clients recovery
from illness
 It occurs when a disease or disability is present or is permanent and
irreversible.
 Interventions are disability limitation and rehabilitation.
Disability limitation:
 Disability is any continuing condition that restricts everyday activities
 Disability is defined as any restriction or lack of ability to perform an
activity in the manner or within the range considered normal for the human
being
 The main aim is to prevent the transition of disability into handicap

 Cardiac or stroke rehabilitation programs, chronic disease management
programs.
Rehabilitation:
 Rehabilitation is the combines and coordinated use of medical, social,
educational and vocational measures for training and retraining the
individual to the highest possible level of functional ability
 Rehabilitation include
i. Medical Rehabilitation – restoration of functional ability
ii. Vocational Rehabilitation - restoration of capacity to earn
iii. Social Rehabilitation - restoration of family and social relationship
iv. Psychological Rehabilitation - restoration of personal dignity and confidence.

DESCRIPTIVE STUDY
1st phase of an epidemiological investigation
Relatively inexpensive and less time-
consuming than analytic studies, they describe,
Patterns of disease occurrence, in terms of,
 When is the disease occurring – time distribution
 Where is it occurring – place distribution
 Who is getting a disease – person distribution

PROCEDURE
• Defining the population to be studied
• Defining the disease under study
• Describing the disease in relation to
• Time
• Place
• Person
• Measurement of disease
• Comparing with known different or same population
• Formulation of etiological hypothesis

Step – 1:
• Defined population means the population not only in
total number, but also its composition in terms of age,
sex, occupation, and cultural characteristics.
• The defined population can be
• whole population
• Sample population
• A selected group – hospital patients , school children
etc
• Communities

Criteria for selecting:
• Large enough
• Stable community
• No migrant or visitor
• Access to medical services.

Step – II: define disease
• The diseases which need to be studied should
be clearly defined.
• The definition of the disease should be precise
and valid.
• An operational definition of disease means of
particular criteria by which the disease is
measured.

Step – III: describe the disease
• Describe the disease in terms of time, place
and person.
TIME DISTRIBUTION
Describe the pattern of disease in terms of
time. A number of questions are raised related
to time distribution. Those questions are

• Whether the disease is occurring season wise ,
moth wise, weekly or yearly.
• Whether there is periodic rise or fall in the
occurrence of disease
• whether the disease follows a consistent time
trend.

Based on the time trend or fluctuation in
disease occurrence disease occur as
i. Short term fluctuations
ii. Periodic fluctuations
iii. Long term fluctuations.

1. Short term fluctuations:
 When the disease occur in population in epidemic
form, then it is short term fluctuations.
 During fluctuations, epidemics are common and
epidemics are of following types
a. Common source epidemic
b. Propagated source epidemic
c. Slow source epidemic.

a. Common source epidemic:
 Occurs from the exposure from one common source.
This exposure can be for a brief period or prolonged
period.
 Depending on this it is of 2 types:
i. Single exposure
ii. Repeated exposure.

i. Single exposure:
 All the individuals who are exposed to the disease
agent for a short period, will develop the disease
within one incubation period of the disease ie.
There is one common source, with one exposure
for short duration to this common source, the
disease will occur among the exposed case.
 So there will be clustering of cases.

ii. Repeated exposure:
 There is common source and on repeated
exposure to this source, the disease will
develop among the exposed individuals.
 This repeated exposure is for prolonged period
which can be continuous or intermittent.
 The epidemics will be irregular, as incubation
period for the cases will not be same.

b. Propagated epidemics:
• Results from person – person transmission of
an infectious agent
• Propagated epidemics occur where large
number of susceptibles are aggregated.
• It also occur where there is regular supply of
new susceptible individuals lowering herd
immunity.

2. Periodic fluctuations:
• The disease occur either in season, months,
days, weeks etc..
• These fluctuations are periodic fluctuations
• These fluctuations occur according to
i. Seasonal trend
ii. Cyclic trend.

i. Seasonal trend:
• Many communicable diseases occur in
particular seasons
• For eg. Upper respiratory infections occur
mostly during winter and GI infections in
summer.

ii. Cyclic trend:
• When the disease spread in cycles over days,
weeks, months, or years, then it is known as
cyclic trend.
• Influenza pandemic occur at an interval of 7 –
10 years.
• automobile accidents more frequently occur on
week ends.

3. Long term fluctuations.
i. Long term fluctuations means the changes in the occurrence
of disease over a long period of time.
ii. It can be several years or decades
iii. There can be either an increase or decrease in the
occurrence of disease
iv. In developed countries, during the past 50 years there is
consistent upward trend in the diseases like coronary heart
disease and diabetes.
v. There is also a decrease in the occurrence of diseases such
as diphtheria and polio.

Place distribution
i. Within countries, the distribution of disease
among population is different due to culture,
standard of living, customs, habits etc..
ii. The changes in the distribution occurs at various
level such as
i. International variations.
ii. National variations.
iii. Rural urban variations.
iv. Local distribution.

i. International variations:
• Disease pattern differs from country to country.
• Breast cancer is low prevalence in japan and highest
prevalence in western countries.
ii. National variations:
• The disease affect some parts of the country more,
while some are affected less and even are not affected at
all.

iii. Rural urban variations:
• Some disease occur frequently in urban areas and some in
rural areas.
• Accidents and drug dependence occur in urban while soil
transmitted and skin diseases are more common in rural
areas.
iv. Local distribution:
• It can be studied with map
• If map shows clustering of cases, it suggests a common
source of infection or all cases are having common risk
factors.

PERSON DISTRIBUTION
 This distribution of the disease in relation to person who
develop the disease should be defined.
 The person developing the disease need to be defined by
i. Age
ii. Sex
iii. Occupation
iv. Habits
v. Social class
vi. Migration
vii. Behavior

Age: Old Age – Arthrosclerosis
Middle Age – Cancer
Childhood – Measles.
Sex: Male – Lung Cancer, Prostate Cancer
Female – Obesity, Diabetes.
Occupation: coal mines – silicosis
Marital status: ca cervix is rare for nuns
Habits : drinking alcohol – cirrohosis of liver.

4. MEASUREMENT OF DISEASE
 As the epidemiologist want to have the clear picture about
the amount of disease in population, so they will use the
measurements such as rates, ratios in terms of death, disease
or disability.
 Descriptive epidemiology can be
 cross- sectional or
 longitudinal design .

5. COMPARING WITH KNOWN INDICES
 To arrive at clues to causative factor of disease,
comparison is made between different population
and sub groups of the same population.

6. FORMULATION OF HYPOTHESIS
 Hypothesis can be formulated related to the
disease etiology.
 Hypothesis is a supposition which is arrived
from observations or reflections.
 The hypothesis is either accepted or rejected,
while formulating hypothesis, the criteria
should be met as given below

 Population
 Specific cause
 Expected outcome
 The amount of cause – effect
 The time period between the exposure to cause
the effect
Eg: The smoking of 30 – 40 cigarettes per day
causes lung cancer in 10% of smokers after 20
years of exposure.

ANALYTICAL STUDY
 Second major type of epidemiology.
 Focus on individual within population unlike
descriptive epidemiology.
 Objective not to formulate hypothesis but to test
hypothesis.
TYPES:
A. Case Control Study
B. Cohort Study

1. CASE CONTROL OR RETROSPECTIVE
DISTINCT FEATURES:
i. Both exposure and outcome have occurred
before the start of disease.
ii. Study proceed backward from effect to cause .
iii. Uses a control or comparison group to support
an inference.

I. SELECTION OF CASES AND CONTROL:
The cases and controls on whom the
study is conducted need to be carefully selected
a. Selection of cases – criteria for selecting:
• There should be clear definition of a case
• The definition of case should meet 2 criteria's ie. Diagnostic and
eligibility criteria.
• For eg. Conducting a study on incidence rate of breast cancer stage I
among females, the cases included in the study will be females,
whose histology report show the presence of cancer and are in I
stage of disease.
• These cases can be taken from hospital or from general population.

b. Selection of controls: The criteria is
• The person should be free from diseases
• Controls should meet other criteria except disease such as
age, sex, occupation etc., the same as cases.
• Controls may be selected from hospitals, relatives,
neighbors and general population.
 Failure to select comparable control can cause “bias”
into the results of case control study.

MATCHING:
Matching means a process of selection of
controls in such a way that they are similar to cases
with regard to certain pertinent selected variables(eg.
Age) which are known to influence the outcome of
disease, if not adequately matched for comparability,
could distort or confounded the result.
For eg. Smoking is confounding factor in the study of role
of alcohol in etiology of esophageal cancer.

Matching can be
i. Group matching
ii. Pair matching.
i. Group matching:
 Within groups, sub categories ie. Strata can be matched for the purpose of
study.
 The strata’s are based on the characteristics such as age, sex, social class,
occupation.
ii. Pair matching:
 Another way of matching is pair ie. For each case, a control is chosen which
can be matched.
 Selecting a 40 year old farmer with particular disease as a case, a 40 year old
farmer without a particular disease as control should be matched.

MEASUREMENT OF EXPOSURE
• The information about exposure should be obtained
both from cases and controls by using the same
methods. These methods are
• Interviews
• Questionnaires
• Studying past record of cases such as hospital records,
employment records etc.
• Clinical or laboratory examination. Investigator should
not know whether a subject is in case or control group.

ANALYSIS AND INTERPRETATION
• The final step is analysis, to find out:
a) Exposure rates
b) Estimation of disease risk

a. Exposure rate:
In case:
• Exposure rate is the ratio of the number of
individuals who became cases after the exposure
to suspected etiological factors to the total
number of cases ( exposed or not exposed to
suspected etiological factor).
exposure rate = a/a+c.

In controls:
• Exposure rate of control can be calculated by
the number of individuals who did not get the
disease even after exposure to suspected
etiological factor to the total number of
individuals who were without the disease.
exposure rate of controls = b/b+d

The frequency rate of lung cancer was
high among smokers than non smokers.
b. Estimation of risk:
• The exposure rate of cases in the study group,
does not mean that all those who exposed to
suspected factor will develop the disease.
• So the estimation of risk is associated with the
exposure, which is calculated by relative risk.

• Odds ratio or cross product ratio:
OR=ad/bc
=33x27/55x2
=8.1
• Smokers shows a risk of having lung cancer 8.1 times
that of non smokers.

ADVANTAGES: CASE CONTROL STUDY
1. Relatively easy to carry out.
2. Rapid and inexpensive
3. Require fewer subjects.
4. Suitable for investigation of rare diseases.
5. No risk of subject.
6. Allows the study of several different etiological factors.
7. Risk factor can be identify
8. No attrition problem because do not require follow up.
9. Minimal ethical problem.

DISADVANTAGES:
1. Problem of bias since it relies on past memory or
past records.
2.Difficulty in selection of appropriate control
group.
3. Can not measure incidence only RR.
4. Doesn’t distinguish between cause and associated
factors.
5.Not suited for the evaluation of therapy or
prophylaxis of disease.

COHORT STUDY
i. It is an analytical study which provide additional evidence
to accept or reject the existing association between
suspected cause and disease.
ii. In epidemiology , cohort is a group of people who share
common characteristics.
iii. The cohort can be of marriage, birth, death or exposure.
Exposure cohort:
 It is group of persons exposed to a common drug or
infection within a defined period.

On the basis of occurrence of disease in
relation to time, cohort studies are of 3 types
i. Prospective study
ii. Retrospective study
iii. A combination of both.

i. Prospective cohort study:
 Start from present and continue into future
 In this type of study, disease in the individual has not
yet occurred at the time investigation starts.
 The outcome ie the disease result in the future.
ii. Retrospective cohort study:
 It is the study in which the outcome (disease) has
occurred before the start of investigations
 It is the study of going back in time
 It can be 10 or 20 0r more years of going back.

iii. Combination of both retrospective and
prospective studies:
In this type of study a group is identified from past
records and is assessed upto present and is
followed up into future for assessment of outcome.

STEPS OF COHORT STUDY
i. Select study subjects
ii. Collect data
iii. Select comparison group
iv. Follow up
v. Analysis

1. Select study subjects:
 The study subjects for cohort study can be selected
either from general population or special groups of
population
2. Collect data:
 The data can be collected by
• Personal interview
• Mailed questionnaires
• Review of records
• Medical tests
• Environmental studies.

3. select a comparison group:
 A comparison group is selected in order to compare morbidity
and mortality rates.
 The comparison group can be in built or the external group or
the general population.
i. Internal comparison:
 The cohort on which the study is conducted is classified into
several comparison groups based on the degree of exposure of
risk factors before the development of disease.
 Comparison group is in built
 For eg. A cohort of exposure who are exposed to risk factor
smoking.

ii. External comparison:
 In some cohort studies, outside comparison group
is required
 The study and control cohorts should be similar in
demographic and other important variables other
than those under study
 Eg smokers and non smokers.

iii. Comparison with general population:
 Sometimes, the study group is compared with
a general population in the same geographic
area as the study groups.
4. FOLLOW- UP
The study need to be followed up depending
upon the type of study
It requires home visits, telephone talks,
observation.

4. FOLLOW- UP
The study need to be followed up depending
upon the type of study
It requires home visits, telephone talks,
observation.
Sometimes it is difficult to achieve 100%
follow up in detail for full duration of study
the reason being the death, change of residence
and migration.

5. ANALYSIS:
The data collected is analyzed in order to find
out the risk rate and frequency of the new
cases in terms of incidence rate.
In incidence rate, the unit of time should be
included
It should include new cases, time period,
population at risk, per 1000 population.

1. Incidence rate =
Number of new cases of specific
disease during a given period X 1000
Population at risk during that period
2. Estimation of risk:
i. Relative risk
ii. Attributable risk.

Relative risk:
incidence of disease among exposed
= incidence of disease among non exposed
= 30 / 2
= 15
• Indicates that incidence rate among exposed is 15
times higher than non exposed
• This value indicates association between cause
and effect.

ii. Attributable risk:
• Calculated by subtracting the incidence of disease rate among non-
exposed from incidence rate of exposed and by dividing this by incidence
rate of exposed and multiple by 100 in order to get the result in
percentage.
• Attributable risk indicate the percentage of disease among sufferer is due
to the causative factor being exposed.
• Eg. Incidence rate of lung cancer exposed is 30 and among non exposed
is 2
• Then AR = 30 – 2/30 x 100
= 93.3%
• 93% of lung cancer among smokers is due to smoking.

EXPERIMENTAL STUDY
Randomized control study:
The steps are
Preparing protocol
Selecting target and study population
Randomization
Intervention
Follow up
outcome

1. Preparing the protocol:
The study is conducted under a strict protocol
which include
 Aims and objective of study
 Statement of problem
 Criteria for selecting groups ie. Study and control group
 Treatment to be used
 standardization of procedures and schedules.

2. Selecting target and study population:
Target population comprise the population of school
children, industrial workers or of a whole city
depending upon nature of the study.
This target population is the reference population
The study population is drawn from the target
population and the study participants in
experimental study.
The study population has the same characteristics as
the target population.

The study population should fulfill the criteria
i. Informed consent should be given by
participants.
ii. Participants should be the representatives of
the target population
iii. Study population should be eligible for the
intervention.

3. Randomisation:
Randomisation is done to eliminate bias and to
allow comparison
Participants are grouped under study and control
group to receive or not to receive interventions.
Randomisation is done after the participants
have given consent and qualified criteria of trial.

4. Intervention:
Intervention is done in the study group by independent
variables such a drugs, vaccines.
The effect of this intervention is measured to have the
outcome.
The outcome is the dependent variable
The effect of drug in the treatment of a particular disease
is either the recovery.
This recovery is dependent variable as it depend on the
independent variable drug ie. Application of drug means
recovery and absence of drug application, leads to no
recovery.

5. Follow up:
Follow up means regular observations and
assessments of both of the groups ie. Control
and study, till the final assessment of the
outcome.
This should be done under same circumstances
in a standard manner for defined interval of time.

6. Assessment:
The outcome of the trial is assessed
The application of independent variable can either cause
positive result or negative result.
Positive results means there will be benefits which will
either cause reduction in severity of disease or reduction
in the incidence rate.
Negative results means intervention, which will cause
more severity of problems and complications
Death opposite to recovery( positive outcome) is the
negative results of the intervention.

RCT is of following types
i. Clinical trials
ii. Preventive trials
iii. Risk factor trial
iv. Cessation experiments
v. Evaluation of health services
vi. Trial of etiological agent.

NON RANDOMISED TRIAL
Non randomised trial is approached in studies,
where RCT approach is sophisticated.
These type of trails are used when due to some
reasons(ethical) it is not possible to conduct RCT
in human beings.
In non randomised control trial there is no
randomisation.
These trials are referred as non experimental
studies.

INTRODUCTION TO EPIDEMIOLOGY

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