EPIDEMIOLOGY -DETAILED

“That branch of
medical science which
treats epidemics"
(Parkin.1873)

“The study of disease,
any disease, as a
mass phenomenon.”
(Greenwood.1934)

“The study of the
distribution &
determinants of disease
frequency in man”(Mac
Mohan,1960)

“The study of the
distribution & determinants
of health related states or
events in specified
populations and the
application of this study to
control of health problems”.

DISEASE FREQUENCY.
DISTRIBUTION OF
DISEASE.
DETERMINANTS OF
DISEASE.

 Refers to the measurement of health
related event in the form of rates &
ratios.
 E.g.. Prevalence rate, Incidence
rates, Death rate etc.
 These rates are essential for
comparing the disease frequency in
different populations or sub groups
of the same population

 Such comparison yield valuable
information on disease etiology.
 This is a vital step in the
development of strategies for
prevention of control of health
problems.

 The basic tenet of epidemiology is
that the distribution of disease
occurs in patterns in a community.
 An important function is to study
the pattern of the distribution in
various subgroups

 Thus epidemiology examines
whether there has been an
increase or decrease over time
span.
 An important outcome of this
step is formulation of etiological
hypothesis

 This aspect of epidemiology is
known as “analytical
epidemiology”.
 Analytical strategies help in
developing scientifically sound
health programmes, interventions
& policies.

 To describe the distribution &
magnitude of health & disease
problems in human populations.
 To identify etiological factors (Risk
Factors)in the pathogenesis of
disease

 To provide the data essential to
the planning, implementation &
evaluation of services for the
prevention, control & treatment
of disease & to the setting up of
priorities among those services.

To eliminate or reduce the
health problem or it’s
consequences.
To promote the health & well
being of society as a whole.

Asking questions.
 Making comparison.

QUESTIONS RELATEDTO
HEALTH EVENTS.
QUESTIONS RELATEDTO
HEALTH ACTION.

 What is the event? (The problem).
 What is the magnitude?
 Where did it happen ?
 When did it happen?
 Who are affected?
 Why did it happen?

 What can be done to reduce this
problem and its consequences?
 How can it be prevented in
future?
 What action should be taken by
the community? By whom these
activities be carried out?

What resources are required?
How are the activities to be
organized?
 What difficulties might arise,
& how might they be
overcome?

 The basic approach in epidemiology is
to make comparison & draw inferences.
 This may be comparison of two or more
groups.
 The first consideration before making .
 comparison is to the “comparability”.
 Matching or randomization helps in
ensuring comparability.

Measurements of mortality.
Measurements of morbidity.
Measurements of disability.
Measurements of natality.

 Measurement of the presence, absence
or distribution of the characteristic or
attributes of the disease.
 Measurement of medical needs, health
care facilities, utilization of health
services & other related events.

 Measurement of the presence,
absence or distribution of the
environmental & other factors
suspected of the environmental
& other factors suspected of
causing the disease.
 Measurement of demographic
variables.

 The epidemiologist usually
expresses disease magnitude as
a RATE, RATIO OR
PROPORTION.

 A rate measures the occurrence of some
particular event in a population during a
given time period.
DEATH RATE = Number of deaths in a year *1000
 MIDYEAR POPULATION

 A RATE COMPRISESTHE
FOLLOWING ELEMENTS.
 Numerator,
 Denominator,
 Time Specification ,
 And a Multiplier

 CRUDE RATES : OR UNSTANDARDIZED
RATES. Eg : Birth rates, Death rates.
 SPECIFIC RATES : Actual observed rates due
to specific causes during specific time
periods.Eg:Tuberculosis – Annual, monthly
rates.
 STANDARDIZED RATES : These are obtained
by direct or indirect method of
standardization.Eg: Age & Sex standardized
rate.

 It expresses a relation in size between two
random quantities.
 The numerator is not a part of the
denominator.
 Ratio is the result of dividing one quantity by
another.
 RATIO = x : y or x
y

 E.g. : The number of children with scabies at
a certain time :
The number of children with
malnutrition at a certain time

 A Proportion is a ratio which indicates
the relation in magnitude of a part of
the whole.
 The numerator is always included in
the denominator.

 A proportion is usually expressed in
percentage.
 E.g.The number of children with
scabies at a certain time *100
The total number of children in the
village at the same time

 Crude Death Rates.
 Specific Death Rates.
 Case Fatality Rates.
 Proportional Mortality Rates.
 Survival Rates.
 Adjusted or Standardized Rates.

 The number of new cases
occurring in a defined
population during a specified
period of time.

 INCIDENCE =
 Number of new cases of specific
disease during a given period
 Population at risk during that time
*1000

 Only to new cases.
 During a given period (one year).
 In a specified population or “population at
risk”, unless other denominators are
chosen.
 It can refer to new spells or episodes of
disease arising.
 Incidence measures the rate at which new
cases are occurring in a population.

 The incidence rates are health status
indicator. It is useful for taking action.
 To control disease.
 For research into etiology.
 For research into pathogenesis,
distribution of disease & efficacy of
preventive & therapeutic measures.
 Provides useful insights into the
effectiveness of the health services
provided.

 Prevalence refers to ALL
CURRENT cases (Old & New)
existing at a given point of time,
or over a period of time in a given
population.
 It is actually a ratio

POINT PREVALENCE.
PERIOD PREVALENCE.

 Point Prevalence is defined as
the number of all current cases
(old & new) of a disease at one
point of time, in relation to a
defined population.

 PP =
 Number of all current cases (old & new) of a
specified disease existing at a given point in
time.

 It measures the frequency of all
current cases ( old & new ) existing
during a defined period of time.
(Annual prevalence).
 It includes cases arising before but
extending into or through to the
year as well those cases arising
during the year.

 Number of existing cases (old & new) of a
specified disease during a given period of
time interval
 Estimated mid interval population at risk
X100

Case 2 Case 3
Case 4
Case 5
Case 7 Case 6
 Case 8
Dec 31
Jan 1
Case 1
Incidence : case 3,4,5 & 8. Point prevalenceJan 1 : 1,2 & 7. Point
prevalence Dec 31 : 1,3,5 & 8. Period prevalence (Jan – Dec) 1,2,3,4,5,7 & 8

 Prevalence depends upon :
 1. Incidence.
 2. Duration of
illness.
 FORMULA : P = I * D
 I = INCIDENCE
 D= MEAN DURATION
 Therefore : I = P/D
 D= P/I

 Descriptive studies are usually the first
phase of an epidemiological
investigation.
 These studies are concerned with
observing the distribution of disease or
health – related characteristics in
human populations.
 Such studies basically ask the questions.

When is the disease occurring?
Where is it occurring?
Who is getting the disease?

 Defining the population to be studied.
 Defining the disease to be studied.
 Describing the disease by ---TIME,
PLACE & PERSON.
 Measurement of disease.
 Comparing with known indices.
 Formulation of an aetiological
hypothesis

 Descriptive studies are investigations of
populations.
 Therefore the first step is to define the
“population base” in terms of total
number, & also by composition in terms
of age , sex, occupation, cultural
characteristics & similar other
information.

 The “defined population” can be the
whole population in a geographic area or
a representative taken from it.
 The defined population should be large
enough so that the specific rates are
meaningful.
 The community chosen should be stable
& not migratory.

 The epidemiologist looks for an
operational definition.
 Operational definition spells out the
criteria by which the disease can be
measured.
 Once operational definition is
established, it should be maintained
through the study.

 Disease is described by person,
place & time distribution.

TIME PLACE PERSON
Year, Season Climatic zones Age, Birth order
Month, Week Country, region Sex, Family size
Day, Hour of
onset
Urban/ rural /
Local
community
Marital State, Height,
Weight
Duration Towns, Cities,
Institutions
Occupation, Social status,
Education, Blood pressure,
Blood cholesterol, Personal

 The pattern of a disease may be
described by the time of occurrence.
 Whether it shows periodic increase?
 Whether it follows a consistent trend?
 Epidemiologists have identified three
kinds of time trends or fluctuations in
disease occurrence.

 SHORTTERM FLUCTUATIONS.
 PERIODIC FLUCTUATION.
 LONG –TERM or SECULARTRENDS

 The best known short term fluctuation
in the occurrence of disease is an
epidemic.
 Epidemic is defined as : the occurrence
in a community or region of cases of an
illness or other related events clearly in
excess of normal expectancy”

A. Common Source Epidemics.
B. Propagated Epidemics.
C. Slow (or) Modern
Epidemics.

 Single Exposure or “POINT
SOUIRCE” epidemics.
 Continuous or “MULTIPLE
EXPOSURE EPIDEMICS”

 A graph of the time distribution of epidemic
cases is called the “EPIDEMIC CURVE”
 An epidemic curve may suggest,
 1. Time relationship with exposure
to a suspected source.
 2. A Cyclical or Seasonal pattern
suggestive of a particular infection

 These are also known as “point Source” epidemics.
 The exposure to the disease agent is brief &
essentially simultaneous, the resultant cases all
develop within one incubation period of the disease.
They are of two types :
 1. Common Source Single Exposure
Epidemics.
 2. Common Source Continuous or
Repeated exposure.


 The exposure to the disease agent is
brief & essentially simultaneous, the
result cases all develop within one
incubation period of the disease.
 E.g., An epidemic of food poisoning

 Common Source Single Exposure
Epidemic curve usually has one peak.
 One point of interest is the “median
incubation period”.
 It is the time required for 50 per cent of
the cases to occur following exposure.

 The epidemic curve rises & falls rapidly ,
with no secondary waves.
 The epidemics tends to be explosive.
 There is clustering of cases over a narrow
interval of time.
 All the cases develop within one incubation
period of disease.

 Common source epidemics are
frequently, but not always due to
exposure to an infectious agent.
 They can result from contamination of
the environment (air, water, food, soil)
by industrial chemicals or pollutants,
E.g., Bhopal gas tragedy in India &
Minamata disease in Japan resulting
from consumption of fish containing
high concentration of methyl mercury

 If the epidemic continues over more
than one incubation period, there is
either a continuous or multiple
exposure to a common source, or a
propagated spread.

 Some times the exposure from the same
source may be prolonged – continuous or
repeated or intermittent – not necessarily at
the same time or place.
 A prostitute may be a common source on
gonorrhea outbreak, but since she will infect
her clients over a period of time there may be
no explosive rise in the number of cases.

 A well of contaminated water or a
nationally distributed brand of vaccine
or food could result in similar outbreaks.
 The outbreak continued beyond the
range of one incubation period.(1976 –
Legionnaire’s disease)
 There was no evidence of secondary
cases among persons who had contact
with ill persons.

 Water borne cholera is a familiar
example, the epidemic reaches a sharp
peak, but tails off gradually over a
longer period of time.

 A propagated epidemic is most often of
infectious origin & results from person to
person transmission of an infectious agent.
 The epidemic usually shows a gradual rise &
tails off over a much longer time.
 Transmission continues until the number of
susceptibles is depleted or susceptible
individuals are no longer exposed to infected
persons or intermediary vectors.

 The speed of spread depends upon herd
immunity, opportunities for contact &
secondary attack rate.
 Propagated epidemics are more likely
to occur where there is a regular supply
of new susceptible individuals lowering
herd immunity.

INITIAL EPIDEMIC HEIGHT OF EPIDEMIC TERMINATION OF
EPIDEMIC

 Two types of periodic fluctuation may
be described.
 1. SEASONALTREND.
 2. CYCLICTREND.

 Seasonal trend is a well known trend in
many communicable disease measles,
varicella, malaria.
 Measles is usually at its height in early
spring.
 Bacterial gastrointestinal infections are
prominent in summer months because of
warm weather & rapid multiplication of
flies.

 Some disease occur in cycles spread over
short periods of time which may be days,
weeks, months or years..
 E.g., Rubella occurred every 6-9 years.
 This is due to naturally occurring
variations in herd immunity.
 A build up of susceptible is again
required in the herd before there can be
another attack.

 Influenza pandemics are known to
occur at intervals of 7-10 years due to
antigenic variations.

 The term “secular trends” implies changes in
the occurrence of disease over a long period
or time, generally several years or decades.
 Secular trend implies a consistent tendency
to change in a particular direction or a
definite movement in one direction.
 E.g., Coronary heart disease, lung cancer
have shown consistent upward trend in the
developed countries during the past 50 years.

 Study of the geography of the disease
(geographical pathology)is one of the
important dimensions of descriptive
epidemiology.
 With the geographical pathology we
gain perspective on the fascinating
differences in disease patterns between
two geographical areas.

 The relative importance of genes versus
environment; changes with migration;
the possible role if diet & other
etiological factors.
 Geographical diseases have profoundly
influenced our understanding of disease,
its nature, its detriments & its relation to
subsequent pathology.

 The geographic variation in disease
occurrence has been one of the
stimulants to national & international
studies.
 The world is not a uniform unit.
 Cultures, standard of living & external
environments vary greatly.The use of
migrant studies is one way to
distinguishing genetic & environmental
factors.

 Geographic patterns provide an
important source of clues about the
causes of the disease.The geographic
variations may be classified as :
 1. International variations.
 2. National variations.
 3. Rural – Urban differences.
 4. Local distributions

 There is a marked international
differences in the occurrence of various
disease.This variations have stimulated
epidemiologists to search for cause –
effect relationships between the
environmental factors & disease.
 The aim is to identify factors which are
crucial in the cause & prevention of
disease

 E.g., Cancer of the oral cavity & uterine
cervix are exceedingly common in India
as compared to industrialized countries.
 There is marked difference between the
incidence of each cancer in different
parts of the world. It is common in Japan,
but unusual in USA.

 Variations in disease occurrence also exist
within countries or national boundaries.
 Distribution of lathyrism, endemic goitre,
flurosis, leprosy, malaria, nutritional
deficiency diseases have all shown
variations in their distribution in India, with
some part of the country more affected than
the others.

 Rural Urban variations are well
known.
 Chronic bonchitis, lung cancer, cardio
vascular diseses, mental illness &
drug dependence are usually more
frequent in in urban areas than in
rural areas.

 On the other hand, skin & zoonotic diseases &
soil transmitted helminths may be more
frequent in rural areas than in urban areas.
 Death rates especially maternal mortality
rates are higher for rural than urban areas.
 These variations may be due to variations in
population density, social class, deficiencies in
health services, levels of sanitation,
education & environmental factors.

 The epidemiologists seek to define
groups which are at higher risk for a
particular diseases, and provides
guidelines to the health administrator
for their prevention & control.

 Inner & outer city variations are
common.
 These variations are best studied with
the aid of “spot maps” or “shaded
maps”.
 These maps show at a glance areas of
high or low frequency, the boundaries &
patterns of disease distribution.

 If the map shows “clustering” of cases , it
may suggest a common source of infection
or a common risk factor shared by all
cases. E.g., John Snow’s cholera epidemic.

 Geographic differences in disease
occurrence is an important dimension of a
descriptive study.These differences are
determined by the agent, host &
environmental factors.

 Large scale migration of human population
from one country to another provides a
unique opportunity to evaluate the role of
the possible genetic & environmental
factors in the occurrence of disease in a
population. Migration studies can be
carried out in two ways :
 1. Comparison of disease & death rates for
migrants with those of their kin who have
stayed at home.

 2. Comparison of migrants with local
population of the host country provides
information on genetically different
groups living in a similar environment.

 In descriptive studies disease is further
characterized by defining the persons who
develop the disease by age, sex,
occupation, marital status, habits, social
class & other host factors.
 These host factors help us to understand
the natural history of disease.

 Age is strongly related to disease than
other host factors.
 Certain factors are more frequent in
certain age groups than others.
 E.g., Measles in childhood, cancer in
middle age & atherosclerosis in old age.
 Many chronic degenerative diseases
show an increasing in the prevalence
with advancing age.

 Some times there may be two separate
peaks instead of one in the age incidence
curve of a disease as in the case of
Hodgkin’s disease, leukaemia & female
breast cancer.
 This phenomenon is known as bio
modality.
 This suggests that two set of causal
factors might be operative.

0 10 20 30 40 50 60 70 80
7
6
5
4
3
2
1
0
incidence

 Gender is another host characteristics which
is often studied in relation disease using
indices such as sex – ratio, sex specific
morbidity & mortality rates.
 It has been found that certain chronic
diseases such as diabetes, hyperthyroidism &
obesity are strikingly more common in women
than in men & lung cancer & CHD are less
frequent in women

 Variations in disease frequency
between sexes have been ascribed to 1.
biological differences including sex
linked genetic inheritance & 2. cultural
& behavioral differences between sexes
(smoking, alcohol, automobile use).

 Differences in disease occurrence have
been noted between population
subgroups of different racial & ethnic
origin.
 These include tuberculosis, essential
hypertension, coronary heart disease,
cancer, & sickle cell anemia.

 Studies related to marital status
reveal that mortality rates were
always lower for married males &
females than for the unmarried, of
the same age & sex.
 Marital status can be a risk factor for
some disease & conditions.

 The observation that cancer cervix is
rare in nuns led to the hypothesis
regarding marital status & cancer cervix.
 Further studies led to the suggestions
that cancer cervix may be associated
with multiple sexual contacts &
promiscuity.

 It is now recognized that man’s
occupation has an important bearing on
his health status.
 Occupation may alter the habit pattern
of an employee e.g., sleep, alcohol,
smoking, night shifts, etc.
 It is obvious that persons working in a
particular occupations are exposed to
particular types of risk.

 Workers on coal mines are more likely
to suffer from silicosis, those in
sedentary occupations face the risk of
heart disease.

 Epidemiological studies have shown
that health & illness are not equally
distributed in social classes.
 Individuals in the upper social classes
have long life & better nutritional
status than those in the lower social
groups.

 Certain diseases like CHD, hypertension,
diabetes have shown a higher
prevalence in upper classes than in lower
classes.
 Social class differences have also been
observed in mental illness & utilization
of medical & health care services.

 Human behavior is increasingly looked upon
as a risk factor in modern day disease such as
coronary heart diseases, obesity & accidents.
 The behavioral factors which have attracted
the greatest attention are cigarette smoking,
sedentary life, over eating & drug abuse.
 Mass movement of people such as pilgrimages
lend themselves to the transmission of
infectious diseases

 Stress has been shown to affect a
variety of variables related to
patients response, e.g., susceptibility
to disease, exacerbation of
symptoms, compliance with medical
regimen, etc.

 In India diseases like leprosy, filaria & malaria
are considered to be rural problems.
 Because of the movement of people from
rural to urban areas these diseases have
created a serious problem in urban areas also.
 Human movement can be classified as 1.
short term, long term & permanent, 2.
according to age, sex, occupation, 3. internal
or external, 4. urban versus rural.

 It is mandatory to have a knowledge on the
disease load in the community.
 This information should be available in terms
of mortality, morbidity, disability & incidence
& prevalence so on.
 Incidence can be obtained from longitudinal
studies & prevalence from cross sectional
studies.


 Are simplest form of an observational
studies.
 It is based on a single examination of a cross
section of population at one point of time.
 Cross sectional studies are also known as
prevalence studies.
 Cross sectional studies are more useful for
chronic diseases

 Cross sectional diseases provide very
little information about the natural
history of the disease or about the
occurrence of new cases.

 In longitudinal studies observations are
repeated in the same population over a
prolonged period of time by means of
follow up examinations.
 Cross sectional studies have been
likened to a photograph, & longitudinal
studies to a cine film

 Longitudinal studies are useful :
 1. To study the natural history of a disease &
its future outcome.
 2. For identifying risk factors of a disease.
 3. For finding out incidence rates or
occurrence of new cases in the
community.
 Longitudinal diseases are more difficult to
organize & more time consuming

 The essence of epidemiology is to make
comparisons & ask questions.
 By making comparisons between
different populations & sub groups of
the same population, it is possible to
arrive at clues to disease aetiology.
 We can also identify the at risk group

 A hypothesis is a supposition, arrived at
from observation or reflection.
 It can be accepted or rejected using the
techniques of analytical epidemiology.

 A hypothesis should specify the
following :
 1.The population.
 2.The specific cause being considered.
 3. Expected outcome – disease.
 4.Time response relationship.

 Provide data regarding the magnitude of
the disease load & types of disease
problems in the community in terms of
morbidity & mortality rates.
 Provide clues to disease aetiology.

 Provide background data for planning,
organizing & evaluating preventive &
curative services.
 Contribute to research by describing
variations in disease occurrence by time,
place & person.

 In contrast to the descriptive studies (
the focus is on the population on the
whole), the analytical studies focus on
individual within the population.
 Analytical studies comprise of two
distinct types of observational studies.
 1. Case Control Studies.
 2. Cohort studies.

Case control studies Cohort studies
•Individuals with
particular disease ---
Cases
•Individuals without
particular disease ---
Controls
PROSPECTIVE COHORT STUDY
Presence or
absence of a
particular
disease
TIME
Factor(s)
present /
Absent
Individual exposed to particular (s)
/ individual unexpected to
particular factor(s)

 Case control studies is often called
“retrospective studies”.
 This is the first approach to test causal
hypothesis.
 The case control method has three
distinct features.

 Both exposure & outcome (disease) have
occurred before the start of the study.
 The study proceeds backwards from
effect to cause.
 It uses a control or comparison group to
support or refute an inference.

 Selection of cases & control.
 Matching.
 Measurement of exposure.
 Analysis & interpretation.

 SELECTION OF CASES :
 Definition of a case is crucial to a case control
study.
 It involves two specifications.1. Diagnostic
criteria. 2. Eligibility criteria.
 DIAGNOSTIC CRITERIA : Specifies the
diagnostic criteria & the stage of the disease.
 ELIGIBILITY CRITERIA : Only newly diagnosed
individuals with in specified time are eligible.

 The sources may be drawn from,
 1. Hospitals.
 2.General Population.
 HOSPITALS : It is convenient to select from
hospitals or network of hospitals, admitted
during a specified period of time.
 GENERAL POPULATION : In a population
based study all cases within in a defined
geographic area are included (entire cases or
randomly selected)

 The controls must be free from the disease
under study.
 They must be similar to the cases as possible,
except for the absence of the disease under
study.
 Selection of an appropriate control group is
an important pre requisite, for it is against
this we will be making comparisons.

 1.Hospital controls.
 2.Relatives.
 3.Neighbourhood controls.
 4.General population.

 The controls may differ from the cases in
a number of factors such as age, gender,
occupation, social status, etc.
 An important consideration is to ensure
comparability between cases & controls.
 This involves what is known as
“MATCHING”

 Matching is defined is to ensure
comparability between cases & controls
in such a way that they are similar to
cases with regard to certain pertinent
selected variables e.g. age, which are
known to influence the outcome of the
disease, or distort or confound the
results.

 A confounding factor is defined as
one which is associated both with
exposure & disease, & is distributed
unequally in study & control groups.

 This may be obtained by interviews,
by questionnaires or by studying
past records of cases & controls -
hospital records, employment
records.

 The final step is analysis, to find out :
 1. Exposure rates among cases &
controls to suspected factor.
 2.Estimation of disease risk associated
with exposure ( odds ratio )

Cases (with Ca lung ) Controls (without Ca lung )
Smokers ( < 5
cigarettes a day )
33 (a) 55 (b)
Non smokers 2 (c) 27 (d)
Total 35 (a+c) 82 (b+d)

 CASES = a
a+c = 33 / 35 = 94.2 %
CONTROLS = b/(b+d) =55/82 = 67 %

 The second analytical step is estimation
of disease risk associated with exposure.
 The estimation of disease risk associated
with exposure is obtained by an index
known as the “Relative Risk” (RR) or
“Risk Ratio”, which is defined as the
ratio between the incidence of disease
among exposed persons & incidence
among non exposed.

 It is given by the formula :
 Relative Risk = Incidence among
exposed
 Incidence among non
exposed
 = a/(a+b)
 c/(c+d)

 Odds Ratio (OR) is the measure of the
strength of the association between risk
factor & outcome.
 The odds ratio is the cross product of the
entries in table.

Exposed
Not exposed
Disease
Yes No
a
c
b
d
Odds Ratio = ad/bc = 33 x 27
55 x 2
= 8.1
Odds ratio is a key parameter in the analysis of case control studies

 Relatively easy to carry out.
 Rapid & expensive.
 Suitable to investigate rare diseases.
 No risk to subjects.
 Allows the study of several different
aetiological factors.
 Risk factors can be identified.
 No attrition problems.
 Ethical problems minimal

 Problems of bias relies on previous records.
 Selection of an appropriate control group may
be difficult.
 We cannot measure incidence,& can only
estimate the relative risk.
 Do not distinguish between causes &
associated factors.
 Not suited to evaluation of therapies.
 Representativeness is a major concern in case
of both cases & controls.

 Cohort study is a type of analytical
study.
 Cohort study is known by a variety of
names: prospective study, longitudinal
study, incidence study & forward
looking study.

 When there is a good evidence of an
association between exposure & disease.
 When exposure is rare, but incidence is
high among the exposed.
 When attrition of the study population
can be minimized.
 When ample funds are available.

Exposed
Not exposed
Disease
Yes No
a
c
b
d
Total
a + b
c + d

 Prospective cohort study.
 Retrospective cohort study.
 Combined of retrospective &
prospective cohort study

 The elements of cohort study are :
 1.Selection of study subjects.
 2.Obtaining data on exposure.
 3.Selectionof comparison group.
 4.Follow up.
 5.Analysis.

 The subjects of cohort study are selected from
general population or select groups within the
population.
 GENERAL POPULATION:
 When the exposure or cause of death is fairly
frequent in the population, cohorts may be
assembled from the general population.
 If the population is large appropriate sample
is taken from the population

 SPECIAL GROUPS :
 These may be special groups that can
readily be studied.(select groups – these
may be professional groups; doctors,
nurses, lawyers, college alumni,
government employees, volunteers, etc.
 These groups are usually homogenous
population

 EXPOSURE GROUPS :
 If the exposure is rare, an economical
procedure is to select a cohort of
persons known to have experienced the
exposure.

 Information about exposure may be
obtained from :
 1.COHORT MEMBERS-personal
interviews, mailed questionnaires.
 2.REVIEW OF RECORDS
 3.MEDICAL EXAMINATION/SPECIAL
TESTS.
 4.ENVIRONMENTAL SURVEYS

 Information about exposure should be
collected in a manner that will allow
classification of cohort members:
 1. according to whether or not they have
been exposed to the suspected factor.
 2. according to the level or degree of
exposure, in the case of special
exposure groups.

Classification of
exposure (cigarettes) No of deaths Death rate
½ pack
1/2 – 1 pack
1 -2 pack
2 packs +
24
84
90
97
95.2
107.8
229.2
264.2

 Comparison may be :
 1.Internal Comparison.
 2.External comparison.
 3.Comparison with the general
population

 INTERNAL COMPARISON :
 The comparison groups are in built.
 Single cohorts enter the study & its
members may, on the basis of
information obtained, be classified into
several comparison groups according to
the degree or levels of exposure to risk
(table - 1)

 EXTERNAL COMPARISON :
 When information on degree of exposure is not
available, it is necessary to put up an external
control, to evaluate the experience of the
exposed group.
 E.g., smokers with non smokers, cohort of
radiologist compared with
ophthalmologist.(The study & control cohorts
should be similar in demographic & possibly
important variables, other than those under
study)

 COMPARISONWITH GENERAL
POPULATION RATES :
 If none is available, the mortality
experience of the exposed groups is
compared with the mortality experience
of the general population in the same
geographic area.

 Follow up is an important aspect of the
cohort study.
 The procedures required comprise :
 1. Periodic medical examination of each
member of the cohort.
 2. Reviewing physician & hospital
records.

 Routine surveillance of death records.
 Mailed questionnaires, telephone calls,
periodic home visits – preferably all
three on an annual basis.

 The data are analyzed in terms of :
 Incidence rates among outcome among
exposed & non exposed.
 Estimation of risk.

Cases (with Ca lung ) Controls (without Ca lung )Cigarette smoking
70 (a) 6930 (b)Yes
3(c) 2997(d)No
Total
7000
(a+b)
3000
(c+d)

 Incidence rates :
 Among smokers = 70/7000 = 10 / 1000.
 Among non smokers = 3/3000 = 3 / 1000.
 Statistical Significance = P<0.001

 ESTIMATION OF RISK :
 RELATIVE RISK : RR is the ratio of the incidence of
the disease or death among exposed & the
incidence among non exposed.
 RR = Incidence among exposed
 Incidence among non exposed.
 RR = 10/1=10
 (it is the measure of the strength of association
between suspected cause & effect

 AR is the difference in incidence rates of
disease between an exposed group & non
exposed group.
 AR = Incidence of disease rate among
exposed minus incidence of disease
rate among non exposed
 Incidence rate among exposed

 10 -1
 10
 Attributable risk indicates to what extent the
disease under study can be attributed to the
exposure.
 90% of the lung cancer among smokers was due
to their smoking.This suggests the amount of
disease that might be eliminated if the factor
under study could be controlled or eliminated
* 100 = 90%

 Population Attributable Risk is the
incidence of the disease in the total
population minus the incidence of
disease among those who were not
exposed to the suspected causal factor

 Heavy smokers = 224 (a) exposed
 Non smokers = 10 (b) non
exposed
 Deaths in total population = 74(c)
 Individual RR = a/b =224/10
=22.40
 Population AR =(c-b)/c = 86%
Deaths per 100,000 persons - years

 Incidence can be calculated.
 Several possible outcomes related to exposure
can be studied simultaneously.
 Provide a direct estimate of relative risk.
 Dose Response ratio can be calculated.
 Certain form of bias can be minimized as
comparison groups are formed before disease
develops

N
o
CASE CONTROL
STUDY
COHORT STUDY
1 Proceeds from effect to
cause
Proceeds from cause to effect
2 Starts with the disease Starts with people exposed to
risk factor or suspected cause
3 Tests whether the
suspected cause occurs
more frequently in those
with the disease than
among those without the
disease
Tests whether disease occurs
more frequently in those
exposed, than in those not
similarly exposed

4 First test to test the
hypothesis
Reserved for testing of
precisely formulated
hypothesis
5 Involves fewer
number of subjects
Involves larger number of
subjects
6 Yields relatively
quick results
Long follow up period

7 Suitable for study
of rare diseases
Inappropriate when
the disease under
investigation is rare
8 Generally yields
only estimate of RR
Yields incidence rates,
RR as well as RR
9 Relatively
inexpensive
Expensive

 Experimental studies are directly controlled &
carried out under the investigator.
 Experimental epidemiology is often equated
with Randomized ControlTrials.
 Experimental studies involve some action,
intervention or manipulation such as
deliberate application or withdrawal of the
suspected cause.

 1.To provide scientific proof of
aetiologic or risk factor.
 2.To provide a method of measuring
the effectiveness & efficiency of
health services for the prevention,
control & treatment of disease &
improve the health of the community

 1. Animal studies.
 2.Human experiments.

 Animal studies have contributed to
our knowledge of anatomy,
physiology, pathology, microbiology,
immunology, genetics, etc.
 Classical animal experiments have
given us a wide range of knowledge.

 Important application of animal
experiments are:
 1.experimental reproduction of human
disease in animals to confirm etiological
hypothesis & to study the patho genetic
phenomenon or mechanisms.

 2.Testing the efficacy or preventive
& therapeutic measures such as
vaccines, drugs.
 3.Completing the natural history of
disease.

 1.Experimental animals can be bred in
laboratories & manipulated easily according
to the wishes of the investigator.
 2.They multiply rapidly & enable the
investigators to carry put certain
experiments (genetic experiments) which in
human population would take several
years..

 1. Not all human diseases can be
reproduced in animals.
 2.All conclusion drawn from animal
experiments may not be strictly
applicable to human beings.There are
difficulties encountered in extrapolating
findings from animal experiments in man

 1. Human experiments are always needed to
investigate disease etiology & to evaluate
the preventive & therapeutic measures.
 2. Before launching human experiments, the
benefits of the experiments have to be
made aware of all possible consequences
of the experiment.

 Experimental studies are of two
types :
 1.RCT.
 2.Non RCT.

 The design of a randomized controlled trial is
as follows.
 The basic steps in conducting a RCT include
the following:
 1.Drawing up a protocol.
 2.Selecting reference & experimental
populations.
 3.Randomization.
 4.Manipulation or intervention.
 5.Follow up.
 6.Assessment of outcome.

 1. Select suitable population
 2. Select suitable sample
 3. Make necessary exclusions
 4. RANDOMIZE
 Experimental group Control
group
 6.Manipulation & follow up
Not eligible
No consent

 The protocol :
 1. Specifies the aims & objectives of
the study.
 2. Questions to be answered.
 3. Criteria for the selection of the
study & control groups.

 4. Size of the sample.
 5.The procedures for allocation of
subjects into the study.
 6.Treatments to be applied.

 7.When &Where & how to what kinds
of patients.
 8.Standardization of work procedures.
 Once the protocol has been evolved, it
should be strictly adhered to
throughout the study. Preliminary runs
may be made to test the feasibility of
the study.

 REFERENCE ORTARGET POPULATIONS:
 1. It is the population to which the
findings of the trial, if found successful,
are expected to be applicable.
 2. A reference population may be as
broad as a general population.

 EXPERIMENTAL OR STUDY POPULATION :
 1.The study population is derived from
the reference population.
 2.It is the actual population that
participates in the study.
 3.Ideally it should be randomly chosen
from the reference population.

 4.They must give an informed consent.
 5.They should be representatives of the
population to which they belong.
 6.They should be qualified or eligible for
the study.
 7. Participants must be fully susceptible to
the disease under study.

 Randomization is a statistical procedure
by which the participants are allocated
into groups usually called “study” &
“control” groups to receive or not to
receive an experimental preventive or
therapeutic procedure, manoeuvre or
intervention.

 1.Randomization is the “heart” of a control
trial.
 2.It will the greatest confidence that the
groups are comparable so that “like can be
compared with like”.
 3.It ensures that the investigator has no
control over the allocation of the
participants to either study or control
group

 4.This eliminates selection bias.(Every
individual gets an equal chance of
being allocated into either group or any
of the trial groups).
 5. Randomization is best done by using
a table of random numbers.

 The experimental group is intervened or
manipulated, by deliberate application
or withdrawal or reduction of the
suspected causal factor. (drug, vaccine,
dietary component as laid down in the
protocol).

 The manipulation creates an
independent variable whose effect is
then determined by measurement of the
final outcome, which constitutes the
dependent variable (incidence of
disease)

 1.This implies examination of the
experimental & control group
subjects art defined intervals of
time, in a standard manner.
 2.The follow up may be short of for a
long time.
 3.There could be problems of attrition.

 1.The final step is to assess the result of
the intervention.
 2.It may be a positive outcome or a
negative outcome.
 3.The positive or negative result is
rigorously compared both with the
experimental & control groups.

 Blinding can be done in three ways.
 SINGLE BLIND :The trial is so planed that
the participant is not aware whether he
belongs to the study or the control group.
 DOUBLE BLINDTRIAL :The trial is so
planned that neither the doctor nor the
participant is aware of the group
allocation & the treatment received.

 TRIPLE BLIND :This goes one step
further.The participant, the investigator
& the person analyzing the data are all
blind.
 Ideally the triple blinding should be used.
 But, most commonly the double blind is
used.

 1. Concurrent parallel study
designs.
 2. Cross over type of study designs.

Random
assignment
Exposed to
specific
treatment
Un exposed
to specific
treatment
Compare Outcome
PATIENTS
Random
assignment
PATIENTS
Exposed to
specific
treatment
Un exposed
to specific
treatment
TIME
Compare Outcome –
Exposed & Un
exposed

 Comparison are made between two
randomly assigned groups, one group
exposed to specific treatment, & the
other group not exposed. Patients
remain in the study group or the control
group for the duration of the
investigation

 In this type of study each patient serves as
his own control
 The patients are randomly assigned to the
control & the study groups.
 The study group receives treatment under
consideration.
 The control group receives a placebo.

 Then patients are in each group are
taken off their medication or placebo to
allow for the possibility of any “carry
over” effects.
 After this period of medication the two
groups are switched.

 1.clinical trials.
 2.preventive trials.
 3.risk factor trials.
 4.cessation experiments.
 5.trial of etiological agents.
 6.evaluation of health services.

 Clinical trials have been concerned
with evaluation therapeutic agents,
mainly drugs.
 A clinical trial is a powerful tool that
is carried out before any new
therapy, procedure or service is
introduced.

 Preventive trial implies trials of
primary prevention measures.
 These trials are purported to prevent
or eliminate on an experimental
basis.
 The most frequently occurring trials
of vaccines

 The basic experimental designs are
applicable for these trials.
 Analysis of a preventive trial must result
in a clear statement about:
 1.The benefit that the community will
derive from the measure.
 2.The risks involved.
 3.The costs to the health services in
terms of man, money, material
resources.

 This is a preventive trial in which the
investigator intervenes to interrupt the
usual sequence in the development of
disease for those individuals who have
“risk factor”, for developing disease.

 In this type of study, an attempt is made
to evaluate the termination of a habit,
(or a removal of the suspected
etiological factors),which is associated
to the causal of disease.
 If such action is followed by a significant
in the reduction in the disease, the
hypothesis if the cause is greatly
strengthened.

 A trial of the etiological agents are
under taken & followed up for the
development of the effect or the
disease.
 E.g.,Retrolental fibroplasia as a cause
of blindness among pre mature babies.

 RCT have been extended to assess the
effectiveness & efficiency of health services.
 Often choices have to be made between
alternative policies of health care delivery
services.
 The necessity of choice results from the fact
that resources are limited & priorities must
be set for the implementation of a large
number of activities which could contribute
to the welfare of the society.

 1.Uncontrolled trials.
 2.Natural experiments.
 3.Before & after comparisons
studies

 1.To study the natural history of disease in
the population.
 2.Community diagnosis.
 3.Planning & evaluation.
 4.Evaluation of individual’s risks &
chances.
 5.Syndrome identification.
 6.Completing the natural history of the
disease.
 7.Searching for causes & risk factors.

 1. Epidemiology aims at closely
studying the diseases load in the
community
 2. Epidemiology provides a means to
study disease profiles & time trends in
human population.

 3.By a study of these trends, we can
make useful projections into the future
& identify emerging health problems &
their correlates.

 4.Epidemiology provides information
about the disease fluctuations in the
community.

 1.Epidemiology helps in diagnosing the
health status of the community.
 2.It quantifies the health related events in
the community in terms of morbidity,
mortality, natality , disability & other
statistics related to the health events
in the population.

 3.By quantification of health problems,
we lay down priorities in disease
control & prevention.
 4.Quantification of morbidity &
mortality serves as a benchmark for the
evaluation of health services. at a later
date.

 5.Quantification of health problems can be a
source of new knowledge about disease
distribution, causation & prevention.
 6.Community diagnosis has been effectively
extended beyond population distributions &
profiles of illness to include an
understanding of the social, cultural &
environmental characteristics of the
community.

 7.Epidemiology therefore has been
described as a “DIAGNOSTICTOOL”

 1.Planning is essential for a rational
allocation of the resources.
 2.Epidemiologic information about the
distribution of health problems over time
& place provides the fundamental basis
for planning & developing the needed
health services.

 3.Epidemiology helps in assessing the
impact of the health services provided to
address people’s problems.
 4.The application of epidemiological
principles to problems of health care
constitutes the “new epidemiology”, E.g.
planning facilities for medical care, man
power planning.

 5.Epidemiology helps in evaluating the
health care services outcome.
 6.The development of RCT has made it
possible to evaluate treatment
modalities on a firm scientific basis.

 1.One of the important tasks of an
epidemiologist is to make a statement
about the degree of risk in a population.
 2.Study of Relative Risk, Attributable
Risk,Population Atributable risk, & other
parameters related of the strength
between cause & association offer us
information on risk status of an individual
& community.

 1.Medical syndromes are identified by
observing frequently associated
findings in individual patients.
 2.Epidemiological investigations can be
used to define & redefine syndromes.

 3.Epidemiological studies have been
able to correct misconceptions
concerning many disease syndrome.

 Epidemiology is concerned with the
entire spectrum of disease in a
population.
 2.The epidemiologist by studying the
disease patterns in the community are
in a better position to fill the gaps in
the natural history of diseases.

 3.Epidemiological investigations yield a
large amount of data on risk factors in
relation to chronic diseases.


 1.Epidemiology by relating disease to
inter population differences & other
attributes of the population tries to
identify the caused of the diseases.
 2.The concept of “risk factors” give us
renewed impetus to epidemiologic
research.

EPIDEMIOLOGY -DETAILED

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