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Introduction to Epidemiology
by
Dr. M.Logaraj, M.D,
Dept. of community medicine

How these inferences are
arrived
 Normal pulse rate 60-90/minute
 Measles & chicken pox mostly occurs during
spring
 Cancer of stomach more common in Japanese
 Lung cancer is common among smokers
 Paracetomol is 90% effective in headache
 Metronidazole is effective against Amoebiasis

Epidemiology
 What is epidemiology ?
 We study health and disease
1. By observing individuals
2. By laboratory of experimental animals
3. By measuring the distribution of health
problem in population
Third one is epidemiology – putting people into
groups

John snow- epidemic of
cholera
 Located the home of each person of cholera
in London 1854
 Found out association between source of
water supply and cholera
 Cholera – spread by contaminated water
before discovery of the organism of cholera

British Doctors Study
 By Doll & Hill
 The relationship between cigarette smoking
& lung cancer in 1950 a follow up study on
British doctors

Epidemiology
 Epidemiology derived from Greek word
 Epi-- on or upon
 demos-- people
 logos-- the study
 The study upon the people or population

Definition
 The study of the distribution and
determinants of health related states or
events in specified population, and the
application of this study to control of health
problems
 Disease frequency
 Disease distribution
 Disease determinants

Disease frequency
 Epidemiology is concerned with the frequency and
pattern of health events in a population
 Measurements of frequency of disease, disability or
death in the form of rates and ratios
 Rates are essential for the comparison of frequency
in different population
 Comparison may yield important clue for the
etiology or formulation of etiological hypothesis

Distribution of disease
 Time characteristics include annual occurrence,
seasonal occurrence and daily or even hourly
occurrence during an epidemic
 Place characteristics include geographic variation,
urban- rural differences.
 Personal characteristics include age, sex, race,
marital status, socioeconomic status, Behaviour and
environmental exposure
 This aspect of study is called as descriptive study

 Descriptive epidemiology provides what,
when, and where of health related events
 What is the event or disease?
 What is the magnitude?
 When did it happen?
 Where did it happen?
 Who are affected?
 The important out come distribution study is
formulation of etiological hypothesis

Determinants of disease
 Test the etiological hypothesis and identify the
underlying causes or risk factors of disease
 This aspect of epidemiology is Analytical
epidemiology
 Which provides why and how of such events by
comparing groups with different rates of disease
occurrence.
 By searching the differences in the characteristics
between the diseased and healthy

Making comparison
 The basic approach in epidemiology is to
make comparison and draw conclusion
 By comparison we try to find out curial
differences in the host and environmental
factors between those affected and not
affected
 Basic tools of measurement are necessary
for the comparison –rates, ratio and
proportion

Rate
 There were 500 deaths from motor vehicle
accident in city A
 In epidemiology compare the rates of
accident in city a with city B
 Rate – elements denominator, numerator,
time specification and multiplier.
 Crude rates
 Specific rates
 Standardized rates

Ratio
 Another measure of disease frequency
 Shows the relation in size between two
quantities
 The numerator not a component of the
denominator
 Sex ratio, doctor population ratio, child
woman ratio etc.

Proportion
 Shows the relation in magnitude of the part of
the whole
 The numerator is always included in the
denominator
 Proportional mortality rate

Measurements of mortality
 Mortality data provides the starting point for
many epidemiological studies.
 Mortality data is relatively easy to collect and
reasonably accurate
 The basis of mortality data is the death
certificate

International death certificate
 For national and international comparison a
standardized system of recording and
classification death
 Part I – deals with immediate
cause( pneumonia) and underlying cause of
death( strangulated hernia)
 Part II – deals with associated disease that
contributed to the death( diabetes)

Limitation of mortality data
 Incomplete reporting of death
 Lack of accuracy
 Lack of uniformity
 Choosing a single cause of death
 Changing coding system
 Diseases with low fatality

Uses mortality data
 Can explain the trends and differences in
overall mortality
 Help in prioritization for health action
 Allocation of scares resource
 For assessment and monitoring of public
health programmes
 Gives important clue for epidemiological
research

Commonly used mortality
rates and ratio
 Crude death rate – simplest measure, lack
comparability
 Specific death rate – age, disease, income, religion
etc.
 Case fatality- killing power of disease for acute and
not chronic
 Proportional mortality rate –cause, age etc. can be
used when population data are not available
 Survival rate- usually for five years
 Standardized rates – direct and indirect

Measurements of morbidity
 Any departure, subjective or objective, from
a state of physiological well-being
 Sickness, illness, disability
Measured by
1. Persons who are ill
2. Illness frequency( spells of illness)
3. The duration

Incidence rate
 The number of NEW cases occurring in a
defined population during a specific period of
time.
Incidence
Number of new cases of specific
Disease during a given time period
Population at risk during that period
X 1000

Uses of incidence rate
 Taking action to control disease
 Research into the etiology or causation
 Research into the pathogenesis
 Studying distribution of disease
 Test the efficacy of preventive and
therapeutic measures
 Used for formulating and testing the
hypothesis

Special incidence rates
Attack rate
 Used only when the population is exposed to
risk for a limited period of time such as during
an epidemic
 Usually expressed as a percentage

Secondary attack rate
 The number of exposed person developing
the disease within the range of the incubation
period following exposure to primary case
SAR
Number of exposed person developing the disease
within the range of the incubation period
Total number of exposed/ susceptible contacts
X100

Secondary Attack Rate
 Limited to application in infectious diseases
 In disease where there are numerous sub-
clinical cases
 Useful to determine the disease of unknown
etiology is communicable or not
 Useful in evaluating the effectiveness of
control measures – immunization

Prevalence
 All current cases ( old or new) existing at a
given point of time or over a period of time in
a given population
 Point prevalence
 Period prevalence

Start of illness
Duration of illness
Incidence - case 3,4,5 & 8
Jan 1 Dec 31
Case 1
Case 2
Case 3
Case 4
Case 5
Case 6
Case 7
Case 8
Point prevalence Jan 1- 1, 2 & 7
Point prevalence Dec 31- 1,3,5 & 8 Period prevalence Jan-Dec- 1,2,3,4,5,7,&8
Number of cases Jan- Dec

Relationship between prevalence &
incidence
• Depends upon two factors, incidence &
duration
P = I X D
Prevalence = incidence X mean duration
Longer the duration of the disease the greater the prevalence
A decrease in incidence & duration will decrease prevalence

• For chronic diseases (TB)- high
prevalence rate relative to incidence
• For acute diseases ( food poisoning,
diarrhoea)- prevalence is relatively low
compared to incidence
• For acute disease- no prevalence ( No of
episodes)
• Treatment decreasing the duration will
decrease the prevalence
• Treatment preventing death but no
recovery will increase the prevalence

Uses of prevalence
• To estimate the magnitude of
health/disease problems in a community
• Identify potential high risk population
• Useful for administrative & planning
purpose ( No of hospital beds, man power
need, rehabilitation facilities)

Aims of epidemiology
 To describe the distribution and magnitude of
health and disease problem in human
population
 To identify etiological factors in the
pathogenesis of disease
 To provide data essential to the planning,
implementation and evaluation of services for
the prevention, control and treatment of
disease and setting up priorities

Epidemiology and Clinical
medicine
Epidemiology Clinical medicine
Population at risk Case or cases
Both sick and healthy Only sick
Relevant data by studying group or
population
History taking Sign and
symptoms Lab investigation
Patient comes to the doctor Investigator goes to the
community
A knowledge of prevalence, etiology and prognosis derived from
epidemiological research is important to the clinician for the
diagnosis and management of individual patient

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Introduction to epidemiology

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