Epidemiology

Epidemiology
SOBANA.M, M.Sc(N).,

DEFINITION
"The study of the distribution and determinants of health related
states or events in specified population and application of this
study to control of diseases and other health problems“
(J.M. Last 1988)

USES OF EPIDEMIOLOGY
• Study of the History of disease.
• Planning
• Community diagnosis
• Assessment of individuals risk.
• Identification of syndrome
• Completion of the clinical picture of
chronic disease
• Searching for causes
• Evaluation.

9/29/2017 4
Scope of epidemiology
Epidemiology covers the various types of field in different types of
activities. It is applied in every field as agricultural, economics, statistics
etc. They are as
• Clinical epidemiology
• Sub-clinical epidemiology
• Infectious disease epidemiology
• Geographical epidemiology
• Social epidemiology
• Chronic disease epidemiology
• Serological epidemiology
• Cancer epidemiology

• Malaria epidemiology
• Neuro epidemiology
• Genetic epidemiology
• Occupational epidemiology
• Psychosocial epidemiology
• Computational epidemiology
• Field epidemiology
• Participative epidemiology
9/29/2017 5

• Molecular epidemiology
• Environmental epidemiology
• Micro- epidemiology
• Macro-epidemiology
• Nutritional epidemiology
• Statistical epidemiology
• Descriptive epidemiology
• Analytical epidemiology
• Experimental epidemiology
• Infectious diseases epidemiology
9/29/2017 6

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.
• This helps to development of strategies for
prevention or control of health related problems.

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 sexes, 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.

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.

Dynamics of disease transmission

Source:
The source of infection defined as the person, animal,
object or substance from which an infectious agent passes
or is disseminated to the host.

Reservoir:
A reservoir is defined as any person, animal, arthropod,
plant, soil or combination of these, in which an infectious
agent lives and multiplies , on which it depends primarily for
survival, where it is reproduce itself in such manner that it can
be transmitted to a susceptible host.
9/29/2017 15

Types of reservoir
Reservoir
Human
reservoir
Cases Carriers
Animal
reservoir
Reservoir of
non-living
things
9/29/2017 16

Cases
Clinical
Sub-
clinical
9/29/2017 17

CARRIERS
By type
Incubator
y
Convales
cent
Healthy
By duration
temporar
y
Chronic
By portal of
exist
urinary Intestinal
Respirato
ry
Skin blood
9/29/2017 18

MODE OF TRANSMISSION
MODE OF
TRANSMISSION
DIRECT
TRANSMISSION
INDIRECT
TRANSMISSION
9/29/2017 19

DIRECT
TRANSMISSION
DIRECT CONTACT
DROPLET
INFECTION
CONTACT WITH
SOIL
INOCULATION
INTO SKIN
TRANSPLACENTAL
TRANSMISSION
9/29/2017 20

INDIRECT
TRANSMISSION
VEHICLE BORNE VECTOR BORNE AIR BORNE FOMITE BORNE
UNCLEAN
HANDS AND
FINGERS
9/29/2017 21

I.SUCCESSFUL PARASITISM
• The infectious agent must find a portal of entry by which it may enter
the host
• Ongoing entry into the host, the organism must reach the
appropriate tissue or site of election in the body of host where it may
find optimum conditions for its multiplication and survival.
• The disease agent must find the portal of exist.
9/29/2017 22

• After leaving the human body the organism must
survive in the external environment for sufficient
period till a new host is found.
9/29/2017 23

II. INCUBATION PERIOD:
• It is defined as the time interval between invasion by
an infectious agent and appearance of the first signs
and symptoms of the disease in question.
9/29/2017 24

EPIDEMILOLOGICAL TRIAD
9/29/2017 25
AGENT
HOST
ENVIRON
MENT

AGENT FACTORS:
• The disease agent is defined as a substance, living
and non-living, tangible or intangible, the excessive
presence or relative lack, which may initiate a disease
process.
9/29/2017 26

Classification of disease agents:
1. Biological agent
2. Nutrient agent
3. Physical agent
4. Chemical agent
5. Mechanical agent
6. Absence or insufficiency or excess of factors necessary
to health
7. Social agent
9/29/2017 27

HOST FACTORS:(INTRINSIC HOST)
In epidemiological terminology, the human host is
referred to as soil and disease agent is seed.
The host factors may be classified as:
1. Demographic characteristics – ( age and gender)
2. Biological characteristics – (genetic factors, biochemical level of
blood, blood groups)
9/29/2017 28

• Social and economic characteristics – (socio-economic
status, education, occupation, marital status, housing)
• Life style factors – personality traits, physical exercise,
use of alcohol/drugs and smoking etc…..
9/29/2017 29

ENVIRONMENTAL FACTORS (EXTRNSIC)
• The environment has been divided into 3
components:
– Physical
– Biological
– psychosocial
9/29/2017 30

MEASUREMENTS OF MORBIDITY AND
MORTALITY
• MORTALITY MEASUREMENTS: (the commonly used measures are;)
1. Crude death rate
2. Specific death rate
A. Specific death rate due to TB
B. Specific death rate for males
C. Specific death rate in age group
D. Death rate for month
E. Weekly death rate
3. Case fatality rate
4. Proportional mortality rate
5. Survival rate
9/29/2017 31

1. Crude death rate:(CDR)
No.of death during the year
CDR=--------------------------------------------X 1000
Mid- year population
9/29/2017 32

2. Specific death rate:
• 1. Specific Death Rate Due To TB:
No.of deaths from TB during a
calendar year
SDR(TB)=------------------------------------------X 1000
Mid-year population
9/29/2017 33

2. Specific Death Rate For Males:
No.of deaths among males during a
calendar year
SDR(M)=-------------------------------------------------X1000
Mid-year population of males
9/29/2017 34

• 3. Specific Death Rate In The Age Group Of 15-20 Years:
No.of deaths of persons aged 15-20
years during a calendar year
SDR=----------------------------------------------------X1000
Mid-year population of
persons aged 15-20 years
9/29/2017 35

4. Death rate for January:
No.of deaths in January X 12
SDR=-----------------------------------------------X1000
Mid-year population
The death rate are multiplied by 12 in order to make the monthly
death rate comparable with the annual death rate.
9/29/2017 36

5. Weekly Death rate :
No.of deaths in a week X 52
SDR=--------------------------------------------X1000
Mid-year population
9/29/2017 37

3. Case fatality rate:
No.of deaths due to particular
disease
CFR=---------------------------------------------------------X1000
Total no.of cases due to same disease
CFR represents the killing power of disease. It is typically used in acute
infectious diseases.
9/29/2017 38

4.Proportional mortality rate:
• It is sometime useful to know what proportion of total deaths
are due to particular cause or what proportion of deaths are
occurring in a particular age group.
• PMR expresses no.of death due to particular cause or in
specific age group per 100 or 1000 total deaths.
9/29/2017 39

A) PMR For Specific Disease:
No.of deaths from the specific disease
in a year
PMR=-----------------------------------------------------X1000
Total deaths from all causes in
that year
9/29/2017 40

B) UNDER FIVE PMR:
No.of deaths under five years of age
in a given year
Under Five PMR=--------------------------------------------x1000
Total no.of deaths during the
same year
9/29/2017 41

C) PMR FOR AGE 50 YEARS AND ABOVE:
No.of deaths of persons aged
50 years and above in a given year
=--------------------------------------------------x1000
Total no.of deaths of all age group
during the same year
9/29/2017 42

5. SURVIVAL RATE:
It is the proportion of survivors in a age group (patients) studied and
follow over a period.
Total no.of patients alive after 5 years
SR=--------------------------------------------------------------X1000
Total no.of patients diagnosed and treated
9/29/2017 43

MEASUREMENTS OF MORBIDITY:
• Morbidity has been defined as any departure,
subjective or objective, from a state of physiological
well being.
• Morbidity commonly measured by 3 aspects:
» Frequency
» Duration
» Severity
9/29/2017 44

1. Disease frequency:
• Disease frequency is measured by incidence and
prevalence rates.
9/29/2017 45

INCIDENCE RATE:
• It is defined as the no.of new cases occuring in a
defined population during a specified period of time.
No.of new case of specific disease
during a given time period
IR=------------------------------------------------------X1000
Population risk during that period
9/29/2017 46

IR REFERS TO:
• Only to new cases
• During a given period ( usually a year)
• In a specified population or population at risk
• It can also refers to spells or episodes of disease
arising in a given period of time per 1000 population.
9/29/2017 47

PREVALENCE RATE:
• The term disease prevalence refers specifically to all
current cases (old and new) existing at given point in
time or over a period of time in a given population.
• Prevalence is of 2 types:
» Point prevalence
» Period prevalence
9/29/2017 48

Point prevalence:
• It is defined as no.of of all current cases (old and
new) of a disease at 1 point of time, in relation to a
defined population.
• The point- a day, several days, week; depending upon
the time it takes to examine the population sample.
9/29/2017 49

No.of all current cases (old and new) of a specified
disease existing at a given point in time
PP=-------------------------------------------------------------X100
Estimated population at the same point
in time
9/29/2017 50

Period prevalence:
• It measures the frequency of all current cases (old and new) existing during a
defined period of time.
No.of all existing cases (old and new) of a specified disease
during a given period of time interval
PP=--------------------------------------------------------------------X100
Estimated mid-interval population at risk
9/29/2017 51

2.DURATION
• The average duration per case or the disability rate,
which is the average no.of days of disability per
person, may serve as a measure of the duration of
illness.
9/29/2017 52

3.SEVERITY:
• The case fatality rate may be used as the index of
severity.
9/29/2017 53

Levels of prevention
• Introduction:
– The successful prevention depends on knowledge of
causation, dynamics of transmission, identification of risk
factors and risk groups, early diagnosis and frequent
measures an organization for applying these measures to
appropriate persons or groups and continuous evaluation of
and development of procedures applied.
9/29/2017 55

There are 4 levels of prevention:
Tertiary
prevention
Secondary
prevention
Primary prevention
Primordial prevention
9/29/2017 56

i. Primordial prevention:
• This is primary prevention in its purest sense.
• It is prevention of the emergence of or development
of risk factors in population group in which they have
not yet appeared
• A new concept, is receiving a special attention in
prevention of chronic diseases.
9/29/2017 57

Example:
• Many adult health problem (obesity, HT) have their
origin in childhood practices like smoking, eating
pattern, physical exercise.
• In primordial prevention, efforts are directed towards
discouraging children from adopting harmful
lifestyles.
9/29/2017 58

Interventions:
• Individual and mass health education
9/29/2017 59
HARMFUL LIFESTYLE
PRACTICES OF
CHILDHOOD
CONTROL
PREVENT FUTURE
HEALTH PROBLEMS

ii. Primary prevention:
• Primary prevention can be defined as action taken
prior to the onset of disease which removes the
possibility of disease will ever occur.
• It signifies intervention in the pre-pathogenesis
phase of a disease or health problems.
9/29/2017 60

Approaches for primary prevention of chronic diseases:
(WHO recommendations)
• 1. Population (mass) strategy
• 2.High risk strategy
1. Population (Mass) Strategy
– Which is directed at the whole population
irrespective of individual risk levels.
2.High Risk Strategy
– It aims to bring preventive care to individuals at
specific risk.
9/29/2017 61

Public health measures and activities of primary
prevention:
• Sanitation
• Infection control
• Immunization
• Protection of food, milk and water supply
• Environmental protection
• Protection against occupational hazards and
accidents.
9/29/2017 62

iii. Secondary prevention:
• It can be defined as action which halts the progress
of a disease at its incipient stage and prevent
complications.
9/29/2017 63

Interventions:
• Early diagnosis
• Adequate treatment
9/29/2017 64

iv. Tertiary prevention:
• It can be defined as all measures available to reduce or limit
impairments and disabilities, minimize suffering caused by existing
departures from good health and to promote the patient’s adjustment
to irremediable conditions.
• when the disease process has advanced beyond it’s early stage , it is still
possible to accomplish prevention by what might be called tertiary
prevention.
9/29/2017 65

Interventions:
• 1. Disability limitation
• 2. Rehabilitation
» Includes medical, psychosocial, vocational rehabilitations.
9/29/2017 66

Levels of
prevention
Primordial
prevention
Primary
prevention
Secondary
prevention
Tertiary
prevention
Modes of
intervention
1. Health
education
(individual &
mass)
1.Health
promotion
2.Specific
protection
1.Early diagnosis
2.Appropriate
treatment
1.Diability
limitation
2.rehabilitation
9/29/2017 67

METHODS OF EPIDEMIOLOGY
• INTRODUCTION:
– The primary concern of the epidemiologist is to study
disease occurrence in people. Epidemiological studies can
be classified into
• 1. OBSERVATIONAL STUDIES
• 2. EXPERIMENTAL STUDIES (INTERVENTIONAL STUDIES)
9/29/2017 69

• Observational Study
– Descriptive studies
– Analytical Studies
• Ecological Study: - Correlation Study unit is a population.
• Cross-Sectional Study: - prevalent Study Individual is a unit
of study.
• Case-Control Study: - case-reference with individual is a
unit of study.
• Cohort study:-Follow up study with individual is a unit of
study.
9/29/2017 70

• Experimental Studies
–Randomized Control Trials
–Field Trials
–Community Trials
9/29/2017 71

OBSERVATIONAL STUDY:
1.DESCRIPTIVE STUDY
• It is descriptive epidemiology/descriptive study.
• It is the first phase of epidemiological investigation
• The studies are concerned with observing the distribution of
disease.
• By:
– Time distribution – when the disease is occurring?
– Place distribution- where it is occurring?
– Person distribution-who is getting the disease?
9/29/2017 72

PROCEDURE IN DESCRIPTIVE STUDY
• Define the population
• Define the disease under study
• Describing the disease by
• TIME
• PLACE
• PERSON
• Measurement of diseases
• Comparing with known indices.
• Formulation of an etiological hypothesis
9/29/2017 73

1.Define the population
• The population can be the whole population in a
geographic area. It can also be a specially selected
group such as age, gender, school children, small
communities.
9/29/2017 74

2.Define the disease under study:
• Define the disease or condition being investigated.
• A definition must be precise and valid to enable
epidemiologist to identify those who have the
disease from who don’t have.
• Operational definition- used to define the disease.
9/29/2017 75

3. Describing the disease
TIME PLACE PERSON
year,
Season
Month
Week
Day
Hours
Duration
Climatic zones
Country
Region
Urban
Rural
Town
Cities
Institution
Age
Gender
Marital status
Birth
Occupation
Social status
Education
Family size
Height
Weight
B.P
Personal habits9/29/2017 76

1. Time distribution:
3 types of time trends/ fluctuations in disease
occurrences.
1. Short term fluctuations – Epidemic diseases
2. Periodic fluctuations
1. Seasonal trend - URTI in summer, GI infections-summer
2. Cyclic trend- cycle spread over short period of time eg:
measles in every 2 – 3 years
3. Long term and secular trend - changes in the
occurrences of disease i.e a progressive increase or
decrease over a long period of time ( years /
decades)
9/29/2017 77

2. Place distribution ( Geographical comparison)
• The variations may be classified as;
– International variations – E.g : CVDs, Cancers.
– National variations – E.g: nutritional deficiencies,
epidemic goiter
– Urban and rural variations – E.g: accidents, lung
cancer, HT- urban area; zoonotic diseases – rural
areas.
– Local distribution- inner and outer city variations.
“ spot maps”/” shaded maps” is used.
9/29/2017 78

3. Person distribution:
• In descriptive study, the disease is further characterized by
defining the person who developed the disease by age,
gender, marital status, occupation, social status etc….
9/29/2017 79

4. Measurement of disease:
• Morbidity and mortality measurements gives clear picture of
disease load.
• Measurement of mortality is straight forward.
• Morbidity has 2 aspects:
1. Incidence – obtained from longitudinal studies
2. Prevalence – obtained from cross sectional
studies
9/29/2017 80

Cross sectional studies:
• It is a single examination of cross-section of
population at one point in time
• The results of which can be projected the whole
population
9/29/2017 81

Longitudinal studies:
• Observation of same population over a prolonged
period of time by means of follow-up examination.
9/29/2017 82

5. Comparing with known indices:
• By making comparison between different population
and sub-group of the same population, it is possible
to arrive at clues to disease etiology.
9/29/2017 83

6. Formulation of hypothesis
• Hypothesis is a supposition, arrived at from observation. An
epidemiological hypothesis should explain;
• Specific cause
• The expected outcome
• The dose response
• The time response
• E.g: smoking causes lung cancer ( incomplete)
• Smoking of 30-40 cigarette/day cause lung cancer in 10% of
smoker after 20 years of exposure – (complete).
9/29/2017 84

Uses of descriptive epidemiology:
• Provide data regarding magnitude and type of disease in
community.
• Provide clues to disease etiology & helps in the
formulation of etiological hypothesis
• Provide background data for planning, organizing &
evaluating preventive & curative services
• The contribute to research by describing variations in
disease occurrence by time, place, person.
9/29/2017 85

II. ANALYTICAL EPIDEMIOLOGY
• Analytical study are the second major type of epidemiological
study.
• The objective is not to formulate but to test the hypothesis.
• The individual subject are evaluated in analytical study.
• 2 types:
– Case control studies
– Cohort studies
9/29/2017 86

1. CASE CONTROL STUDIES:
• It is often called retrospective studies.
• Features of case control studies:
– Both exposure and outcome have occurred before the
start of study
– The study proceeds backwards from effect to cause
– it uses a control or comparison group to support or
refute an inference
9/29/2017 87

• By definition a case control study involves two
population
• cases and controls
• In CCSs the unit is individual rather than the group.
• The focus is on disease that has already developed
9/29/2017 88

Basic steps in case control study:
• There are 4 basic steps :
– Selection of cases and controls
– Matching
– Measurement of exposure
– Analysis and interpretation
9/29/2017 90

i) Selection of cases and controls:
• Selection of cases:
– Cases are selected based on diagnostic criteria
and eligibility criteria.
• Sources of cases:
– Hospital
– General population
9/29/2017 91

Selection of controls:
• The controls may be free from disease under study.
These may be as similar to the cases as possible,
except for the absence of disease under study
• As a rule, a comparison group is identified before a
study is done.
• Sources of controls:
• Hospitals, relatives, neighbors and general population.
9/29/2017 92

ii) matching:
• The control may differ from cases in a no.of factor
such as age, gender, occupation & social status. An
important consideration is to ensure comparability
between cases and controls. This is known as
matching.
9/29/2017 93

iii) Measurement of exposure:
• Information about exposure should be obtained
precisely the same manner both for cases and
controls.
• This may be obtained by interviews, questionnaires,
studying of past record of cases such as hospital
records and employment records.
9/29/2017 94

iv) Analysis :
• The final step is analysis to find out
• Exposure rate among cases and controls to
suspected factors
• Estimation of disease risk associated with
exposure
9/29/2017 95

2.COHORT STUDY
• In epidemiology the term cohort is defined as a
group of people who share a common characteristics
or experience within a defined time period.
– Example: age cohort, occupational cohort,
marriage cohort, pregnancy cohort, birth cohort.
9/29/2017 96

• Cohort study is known by variety of names:
• Prospective longitudinal study
• Incidence study
• Forward looking study
• The features of cohort studies:
– Cohorts are identified prior to the appearance of
disease under investigation.
– The study group observed over a period of time to
determine the frequency of disease among them.
– The study proceeds forward from cause to effect.
9/29/2017 97

Types of cohort studies:
• Prospective cohort study
• Retrospective cohort study
• Combination of retrospective and prospective cohort
study
9/29/2017 99

Prospective or current cohort study:
• It is one in which the outcome has not yet occurred
at the time the investigation begins.
9/29/2017 100

Retrospective cohort study:
• Or historical is one in which the outcome have all
occur before the start of the investigation.
9/29/2017 101

Combination of retrospective and prospective
cohort study
• In this type of study both the prospective and retrospective
elements are combined. The cohort is identified from past
records and the same cohort is followed prospectively in
future for further assessment outcome.
9/29/2017 102

Steps of cohort study:
• Selection of study subjects
• Obtain data on exposure
• Selection of comparison group
• Follow-up
• Analysis
9/29/2017 103

1. Selection of study subjects
• General population
• Special groups
• Select groups
• Exposure groups
• Select groups
– These may be professional groups ( doctors/nurses), insured
persons, college alumni, government employee and volunteers
etc…
• Exposure groups
– Cohort may be selected because of special exposure to physical,
chemical or other disease agent.
9/29/2017 104

2. Obtaining data on exposure:
• Information about exposure may be obtain directly from
cohort members through personal interviews, mailed
questionnaires, preview of records, medical examination or
special tests, environmental surveys.
9/29/2017 105

3. Selection of comparison groups:
• There are many ways of assembling comparison
groups:
– Internal comparison
– External comparison
– Comparison with general population
9/29/2017 106

• Internal comparison: In some cohort studies, no outside
comparison group is required. The comparison groups are inbuilt
within the individual study itself.
• 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 groups.
• Comparison with general population: 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 as exposed peoples.
9/29/2017 107

4. follow-up:
• One of the problem of cohort study is the regular follow-
up of all the participants.
• The procedure required comprises of:
– Periodical medical examination of each cohort
members
– Reviewing physician and hospital record
– Routine surveillance of death records
– Mailed questionnaires, telephonic calls
– Periodic home visit
9/29/2017 108

5. Analysis :
• The data are analyzed in terms of
–Incidence rates of outcome among exposed
and non-exposed
–Estimation of risk associated with disease.
9/29/2017 109

Experimental Studies Or Experimental
Epidemiology:
• These are also called as interventional studies.
• Experimental studies involves some actions, interventions or
manipulation such as deliberate application or withdrawal of the
suspected cause in the experimental group while making no change
in the control group, and observing & comparing the outcome of
the experiment in both the groups.
9/29/2017 110

Aims :
• To provide scientific proof of etiological factors
• To provide a method of measuring the effectiveness and
efficiency of health services for prevention, control and
treatment of disease and improve the health of the
community.
9/29/2017 111

Types of experimental studies:
1. Randomized Controlled Trials
2. Non-randomized Controlled Trials
9/29/2017 112

Randomized controlled trials (RCT)
• Those involving the process of random allocation
• Randomization means- giving equal chance to all
samples
9/29/2017 113

The basic steps in conducting RCT:
1. Drawing up of protocol
2. Selecting reference and experimental population
3. Randomization
4. Manipulation or intervention
5. Follow-up
6. Assessment of outcome
9/29/2017 114

1. Drawing up of protocol
• One of the essential features of RCT is the study is conducted
under strict protocol.
• The protocol specifies the aims and objectives of the study,
questions to be answered, criteria for the selection of study
groups and control groups, size of the sample, the procedure for
allocation of subjects into study and control groups, treatment to
be applied when and where?, standardization of working
procedures and schedules and responsibilities of researcher,
evaluation of outcome of the study.
9/29/2017 115

2. Selecting reference and experimental population
 Reference / target population: it is the population to which the
findings of the trials are expected to be applicable.
 Experimental or study population: it is derived from target
population. It is the actual population that the participants in the
experimental study. It should be randomly chosen from the
reference population
9/29/2017 116

3. Randomization :
• Randomization is a statistical procedure by which the
participants are allocated into groups usually called study
or control groups, to receive or not to receive an
therapeutic procedure or intervention.
• Randomization is an attempt to eliminate “bias”
9/29/2017 117

4. Manipulation / Intervention:
• Manipulation means doing something.
• Having formed the study and control groups, the next step is
to intervene / manipulate the study(experimental) group by
deliberate application or withdrawal of procedure.
9/29/2017 118

5. Follow-up :
• This implies examination of the experimental and control
group subjects at defined interval time, till the final
assessment of outcome.
• The duration of the trial is usually based on the expectation
that a significant different will be demonstrated at a given
point in time after the start of the study.
9/29/2017 119

6. Assessment Of Outcome:
• The final step is assessment of the outcome of the trial in
terms of ;
• Positive results
• Negative results
• Positive results – i.e benefits of the experimental measures.
• Negative results – i.e severity and frequency of side-effects
and complications, including death.
9/29/2017 120

2. Non- Randomized Controlled Trials:
• Although the experimental method is almost always to be preferred, it is
not always possible for ethical, administrative and other reasons to
randomized controlled trials in human beings.
• Example:
– Smoking and lung cancer
9/29/2017 121

• The cost and logistics are often prohibited
• These trials are rare
In such situations, we must depend upon non-
randomized trials designs.
Examples of NRCTs:
1. Uncontrolled trials
2. Natural experiments
3. Before and after comparison studies
9/29/2017 122

1.Uncontrolled Trials
• Uncontrolled trial is the trials with no comparison groups.
• Eg : screening of cervical cancer by PAP smear test is effective in
reducing mortality from this disease.
• Initially the uncontrolled trials useful in evaluating specific therapy
on particular diseases, appropriate dose, investigate adverse
reactions.
9/29/2017 123

2. Natural experiments
• Where experimental studies are not possible in
human population, the epidemiologist seeks to
identify “ natural circumstances” that mimic an
experiment.
• Eg: a major earth quake in Athens in 1981 provided a
natural experiment to epidemiologist to study the
effect of acute stress on cardiovascular mortality.
9/29/2017 124

3. Before and after comparison studies
• Before and after comparison studies without
controls.
• Before and after comparison studies with controls.
9/29/2017 125

Before and after comparison studies
without controls.
• These studies centered round comparing the
incidence of disease before and after introduction of
preventive measures.
• The events which took place prior to the use of new
treatment or preventive procedure are used as a
standard for comparison.
9/29/2017 126

Before and after comparison studies with
controls.
• In the absence of control group, between before and after the use
of new treatment or procedure may be misleading. In such
situations the epidemiologist tries to utilize a natural control group
i.e one provided by the nature or natural circumstances.
• If preventive programme is to be applied to an entire community
we would select another community as similar as possible.
9/29/2017 127

Reference :
• K Park (2015) Park's textbook of preventive and social
medicine , Twenty-third edition, Bhanot Publishers, India.

Epidemiology

More Related Content

What's hot (20)

Similar to Epidemiology (20)

Recently uploaded (20)

Epidemiology