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Introduction of epidemiology
The Greek physician Hippocrates is sometimes
said to be the father of epidemiology. He is the
first person known to have examined the
relationships between the occurrence of disease
and environmental influences.
• epi – means “on, upon, befall”
• demo – means “people, population, man”
• logy – means study
Definition of Epidemiology:-
The study of frequency, distribution and
determinants of diseases is known as
Key Words of Epidemiology
• Epidemic : ( Epi = upon : demos = people )
An outbreak of disease in a community in excess of
“normal expectation ”
• Endemic: (en = in; demos = people). The
constant presence of disease within a geographic
area or the usual prevalence of a given disease in a
particular area. malaria, tuberculosis, etc.
• Pandemic: (pan = all: demos = people) An
epidemic which spreads from country to country or
over the whole world, as for example, the recent
epidemic of AIDS.
Aims of Epidemiology
•To describe the distribution and size of disease
problems 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 disease and
to setting up the priorities among these services.
Uses of Epidemiology
•To study the effects of disease state in a
population over a time and predict future health.
• To diagnose the health of the community.
• To evaluate health services.
• To estimate the individual risk from group
•To identify the syndrome.
•To search for causes of disease.
Levels of Prevention
Early diagnosis (screening tests, case finding programmes )
– Functional rehabilitation
– Vocational rehabilitation
– Social rehabilitation
– Psychological rehabilitation
1. Asking questions
2. Making comparisons
1. Asking questions
Epidemiology has been defined as a means of
learning or asking questions.. and getting answers that lead
to further questions
RELATED TO HEALTH EVENTS
•What is the event? (the problem)
•Where did it happen?
•When did it happen?
•Who are affected?
•Why did it happen?
RELATED TO HEALTH ACTIONS
•What can be done to reduce this problem and its
•How can it be prevented In the future ?
•What action should be taken by the community ? By
the health services? By other sectors ?
•What resources are required ? How are the activities to
be organized ?
2. Making comparisons
•This approach is to make comparisons and draw
•Comparison may be made between different
population at a given time eg. Rural with urban
•Between sub group of population eg. Male with
•Between various periods of observation eg.
Tools of measurement
A rate measures the occurrence of some particular
event (development of disease or the occurrence of
death) in a population during a given time period.
Number of deaths in one year
Death rate = X 1000
Mid - year population
(1) Crude rates: These are the actual observed
rates such as the birth and death rates. Crude rates are
also known as unstandardized rates.
(2) Specific rates: These are the actual observed
rates due to specific causes (e.g., tuberculosis); or
occurring in specific groups (e.g., age-sex groups) or
during specific time periods (e.g.. annual, monthly or
(3) Standardized rates: These are obtained by
direct or indirect method of standardization or
adjustment, e.g., age and sex standardized rates.
Another measure of disease frequency is a
ratio. It expresses a relation in size between two
•The number of children with malnutrition at a
• sex-ratio, doctor population ratio, child woman
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 as a percentage.
The number of children with
scabies at a certain time
Example x 100
The total number of children in
the village at the same time
The occurrence of death in a population.
Mortality data are relatively easy to obtain,
and, in many countries, reasonably accurate. Many
countries have routine systems for collecting
mortality data each year, information on deaths is
analyzed and the resulting tabulations are made
available by each government.
Limitations of mortality data
• Incomplete reporting of deaths
• Lack of accuracy:-inaccuracies in the recording of
age and cause of death
• Lack of uniformity:- There is no uniform and
standardized method of collection of data
• Changing:- Changing coding systems and changing
fashions in diagnosis may affect the validity.
• Diseases with low fatality:-the disease is associated
with low fatality (e.g., mental diseases, arthritis).
MORTALITY RATES AND RATIOS
1. Crude death rate
The simplest measure of mortality is the crude
death rate It is defined as the number of deaths
(from all causes) per 1000 estimated mid-year
population in one year, in a given place.
Number of deaths during the year
= X 1000
2. Specific death rates
When analysis is planned to throw light on
aetiology, it is essential to use Specific death
rates. The specific death rates may be
(a) cause or disease specific e.g., tuberculosis
(b) related to specific groups e.g. age-specific,
sex-specific , age and sex specific, etc.
3. Case fatality rate (Ratio)
Case fatality rate represents the killing power of a
disease.it is simply the ratio of deaths to cases. The time
interval is not specified. Case fatality rate is typically used
in acute infectious diseases (e.g., food poisoning, cholera,
Total number of deaths due to
a particular disease
= X 100
Total number of cases due to
the same disease
4. Proportional mortality rate (Ratio)
•It is sometimes useful to know what proportion of total
deaths are due to a particular cause (e.g.. cancer) or
•what proportion of deaths are occurring in a particular
age group (e.g., above the age of 50 years).
•Proportional mortality rats expresses the number of
deaths due to a particular cause (at in a specific age
group) per 100 (or 1000) total deaths. Thus we have:
(a) Proportional mortality from a specific disease
Number of deaths from the specific
disease in a year
= X 100
Total deaths from all causes
in the year
2. Under 5 proportionate mortality rate
Number of deaths under 5 years of age
in the given year.
= X 100
Total number of deaths during
the same period
3. Proportional mortality rate for aged 50 years and above
Number of deaths at persons aged 50
years and above
= X 100
Total deaths at all age groups
in that year
It is the proportion of survivors in a group, (e.g. of
patients) studied and followed over a period tag, a 5 year period.
Survival rates have received special attention in cancer studies.
Total number of patients alive
after 5 years
Survival rate = X 100
Total number of patients diagnosed
The occurrence of an illness or illnesses in a
Morbidity has been defined as any departure, subjective or
objective, from a state of physiological well-being. The term
is used equivalent to such terms as sickness. illness,
WHO Expert Committee on Health Statistics noted in
Report that morbidity could be measured in terms of 3
units (a) persons who were ill
(b) the illnesses (periods or spells of illness) that these
persons experienced; and
(c) the duration (days, weeks, etc.) of these illnesses
Incidence rate is defined as the number of NEW
cases occurring in a defined population during a specified
period of time . It is given by the formula :
Number of new cases of specific disease
during a given time period
Incidence = X 1000
Population at risk during that period
For example:- if there had been 500 new cases of an illness
in a population of 30,000 in a year, the incidence rate would
= 500/30,000 x 1000
= 16.7 per 1000 per year
The term "disease prevalence" refers specifically
to all current cases (old and new) existing at a given
point in time, or over a period of time in a given
(a) Point prevalence (b) Period prevalence
(a) Point prevalence
•Point prevalence of a disease is defined as the number
of all current cases (old and new) of a disease at one
point of time, in relation to a defined population. several
days, or even a few weeks.
Point prevalence is given by the formula:
Number of all current cases (old and new) of a
specified disease existing at a given point in time
Estimated population at the same point in time
(b) Period prevalence
A less commonly used measure of prevalence is period
prevalence. It measures the frequency of all current cases (old
and new) existing during a defined period of time (e.g., annual
prevalence) expressed in relation to a defined population. cases
arising during the year.
Period prevalence is given by the formula:
Number of existing cases (old and new) of a specified
disease during a given period of time interval
Estimated mid interval population at risk
METHODS OF EPIDEMIOLOGICAL
Epidemiologists employ three different
methods or approaches for epidemiological
studies which are:
1. Descriptive method
2. Analytical method.
3. Experimental method
1. Descriptive Method
Descriptive method of epidemiological study
is concerned with the study of frequency and
distribution of disease and health related events
in population in terms of person, place and time.
This method gives information about which
all are affected by a particular disease or health
related event or problems, where the cases occur
and when they occur. The data is collected about:
• Personal characteristics such as age, sex, race,
marital status, occupation, education, income,
Class, dietary pattern, habits.
• Place distribution of cases i.e. areas of high
concentration, low concentration and spotting in
• Time distribution/trends such as year, season,
month, week, day and hour of onset of the
Such information’s give clue to possible
associated factors such as age with specific
disease e.g. measles, diphtheria, pertusis in early
childhood, cancer in middle age, arterioscleroses
in old age, some habits like smoking with lung
cancer, dietary pattern with obesity. The data
collected are analyzed and presented in terms of
percentages, rates and ratios.
- Cross-sectional studies
In this design of descriptive method of study, the data is
collected from a cross-section of population at a one
point in time.
The results of the study are applied on the population. The
cross-section of the population is sampled carefully so
that it is representative of the whole population.
Cross-section study is like a snapshot and provides
information about the prevalence of a disease.
It is also called as prevalence study. example study of
diabetes or hypertension by personal characteristics and
- Longitudinal Studies
Longitudinal studies art useful for studying
the natural history of diseases, finding out
incidence rates of diseases and identifying risk
factors of diseases. Longitudinal studies are
more expensive and time consuming than cross-
2. Analytical Method
You have learn that descriptive studies
generate etiological clues for various diseases
help in formulating a guess or hypothesis for
further vigorous study or testing e.g. "Cigarette
smoking (10 to 20 in a day) causes lung cancer in
10 to 15% of smokers after 20 years of
exposure”, wife battering is related to violence in
victim's childhood family of origin". These types
of hypothesis are further studied and tested by
analytical studies to determine the association of
cause with the effect.
- Case control study
In this method a group of people who have
been diagnosed as having a particular problem
e.g. lung cancer (cases) are compared with a
group of people who are similar in
characteristics to that of cases but they are free
from the problem i.e. free from lung cancer
under study (controls).
- Cohort Study
A cohort is a specific group of people, at a
certain time, sharing common characteristics or
experience e.g. people born on the same day or
the same year (birth cohort), couple married in
the same year (marriage cohort), a class of
nursing students (experience cohort), people
With same occupation (occupation cohort) etc.
a) Prospective cohort study: Cohort study is
prospective in nature because the group under study
is free from the disease but exposed to risk factor
and epidemiologist study the development of a
condition over time. example-lung cancer
b) Retrospective cohort study: This is also called as
historical cohort study. In this type of study designs
the event has already occurred
• Experimental studies are similar in approach to cohort
studies except that conditions are under the careful
control of investigator.
• Experimental studies are done to confirm the etiology of
diseases,' establish the efficacy of preventive or
therapeutic measures and evaluate health care services.
• These' studies are done under controlled conditions.
• The investigator administers an intervention/gives
treatment to the experimental group which is either
exposure to causative factor of disease or
preventive/therapeutic measure to improve or influence
health or prevent disease but not to the control group
which is similar to experimental group in all its aspects.