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Based on two fundamental assumptions:
o First, human disease does not occur at random
o Second, human disease has causal and preventive
o Epidemiology is define as the study of the
distribution and determinants of health-related states
or events in specified populations and the
application of this study to control of health
o There are three closely interrelated components:
distribution, determinants and frequency and it
encompass all epidemiological principles and
Last JM: A Dictionary of Epidemiology, Edition 2. New York, Oxford University Press, 1988.
o The measurement of disease frequency
involves quantification of the existence or
occurrence of disease.
o The availability of such data is a prerequisite
for any systematic investigation of patterns of
disease occurrence in human populations
o The distribution of disease considers such questions as
o who is getting the disease within a population
o where and when the disease is occurring
o (TPP: time, place, persons).
o Knowledge of such distribution is essential to describe
patterns of disease as well as to formulate hypothesis
concerning possible causal and / or preventive factors
o The determinants of disease is derived from
the first two; since knowledge of frequency
and distribution of disease is necessary to test
an epidemiological hypothesis
o Epidemiology contributes to the rationale for public
health policies and services and is important for use
in their evaluation.
o But the delivery of those services or the
implementation of those policies is not part of
epidemiology (Savitz et al., 1999: 1158-1159)
Functions of Epidemiology
o Discover the agent, host, and environmental factors
that affect health, in order to provide the scientific
basis for the prevention of disease and injury and
the promotion of health.
o Determine the relative importance of causes of
illness, disability, and death, in order to establish
priorities for research and action.
o Identify those sections of the population which have
the greatest risk from specific causes of ill health, in
order that the indicated action may be directed
o Evaluate the effectiveness of preventive and
therapeutic health programs and services in
improving the health of the population.
*Milton Terris, The Society for Epidemiologic Research and the future of epidemiology. Am J Epidemiology 1992; 136(8):909-915, p 912
Aims of Epidemiology
o To describe the distribution and magnitude of health
and disease problems in human populations
o To identify aetiological factors (risk factors) in the
pathogenesis of disease
o To provide the data essential to the planning,
implementation and evaluation of services for
prevention, control and treatment of diseases
Basic Measurement in Epidemiology
Tools of Measurement
o Relative Risk
o Odds Ratio
Uses of Epidemiology
o To study historically the rise and fall of disease in
o Community diagnosis
o Planning and evaluation
o Evaluation of individual’s risks and chances
o Syndrome identification
o Completing the natural history of disease
o Searching for cause and risk factor
o Tools to quantify how common an illness is in a
population in a time.
o Case (event/outcome of interest)
o Size of a population (the population at risk)
o Measurement of disease frequency is a prerequisite
for any epidemiologic investigation
o Epidemiology is the study of the distribution
and determinants of health-related states or
events in specified populations in a given time,
and the application of this study to the control
of health problems.
o To achieve either of these objectives it is first
necessary to measure disease frequency.
o Simply counting the affected individuals is not
o Population at risk has to be identified.
Ex: Men should not be included in calculations of
frequency of carcinoma cervix. Frequency of brucellosis
to be measured only in people working in farms and
o Prevalence Rate
o Incidence rate
o Case fatality rate
o Mortality rates(age specific/cause specific)
o Attack rate
RATIO: A fraction in which the numerator is not part of the
Ex: Fetal death ratio: Total no. of fetal deaths/total no. of live
Fetal deaths are not part of live births
PROPORTION: A fraction in which the numerator is part of
Ex: Proportional mortality.
o Most fractions in epidemiology are proportions.
RATE: A proportion in which change over time
- But in practice, the term “ rate” is often used
interchangeably with ratio without reference
Ex: fetal death rate & fetal death ratio, maternal
mortality rate & maternal mortality ratio.
Prevalence vs. Incidence
oPrevalence: frequency of existing cases
o Incidence: frequency of new cases
o New cases are called incident cases.
o Existing cases are called prevalent cases.
No. of people with disease at specified time
No. of people in Population at risk at specified time
expressed as cases per 1000/ 10,000
o Point prevalence: Proportion of a population
affected by a disease at a given time.
o Period Prevalence: Proportion of individuals in
a specified population at risk who have the
disease of interest over a specified period of
Ex: annual prevalence rate.
(When the type of prevalence rate is not
specified it is usually point prevalence, or its
closest practical approximation)
Increased by :
longer duration of the disease
prolongation of life without cure
increase of new cases( i. e. incidence)
in-migration of cases
out-migration of healthy people
in-migration of susceptible people
improved diagnostic facilities
oShorter disease duration
oHigh case fatality from disease
odecrease in new cases
oIn-migration of healthy people
oOut-migration of cases
oImproved cure rate of cases
o Since prevalence rates are influenced by so many
factors unrelated to disease causation, do not usually
provide strong evidence of causality
o Prevalence rates are used to measure the occurrence
of chronic conditions. Ex: Diabetes, Rheumatoid
arthritis and assessing the health care needs & health
Ex: The percentage of people with malaria parasite in
their blood in a village in Chandragadhi in a
survey in December 2007 and the Percentage of
under five children with acute malnutrition in
Humla in March 2008
Incidence Rate is defined as the no. of NEW cases occurring in a
defined population during a specified time period.
No. of new cases of specific disease during a given time period
Population at risk during that period
Prevalence = Incidence x Duration of disease
o Larger studies: mid-period population
o Also called incidence density or force of morbidity
o Expressed as number of new cases per person-time at
o Person-time can be person-days, person-months, person
years, but more common is per 100 person-years
o 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 be :
500/30000 x 1000 = 16.7 / 1000 per year
o Incidence rate must include the unit of time
used in final expression
o Incidence rate refers to only new cases.
The probability that an event will occur
if during outbreak called Attack rate
Number of new cases with disease in a specified time period
CI = ---------------------------------------------------------Number of disease-free people at the start of the time period
Disease-free persons are Population at risk
Attack Rate (AR)
o Cumulative incidence during an outbreak
o Usually expressed for the entire epidemic period,
from the first to the last case
o Not really a rate but a proportion
Ex: Outbreak of cholera in country X in March 1999
Number of cases = 490, Population at risk = 18,600
Then the Attack rate = 2.6%
Case Fatality Rate
• Measure of the severity of a disease which defined
as the proportion of cases of a specified disease or
condition which are fatal within a specified time
= no. of death from a disease in a specified period
no. of diagnosed cases of disease in same period
Cause specific mortality rate
o No. of deaths from a specific disease
in a population in a given period
Mid year population
o Cause specific mortality rate can be used for certain
age groups for example in under 5 mortality , the
common causes are ARI or Diarrhea.
Proportional Mortality Rate
o It expresses the no. of death due to a particular cause (or in
a specific age group) per 100 (or 1000) total deaths
o Proportional mortality for a specific cause:
No. of deaths from the specific disease in a year
Total deaths from all causes in that year
o Maternal mortality ratio (MMR): The number of women who die as a result
of pregnancy and childbirth complications per 100,000 live births in a given
o Crude death rate: Annual number of deaths per 1,000 population.
o Crude birth rate: Annual number of births per 1,000 population.
o Under-five mortality rate: Probability of dying between birth and exactly
five years of age expressed per 1,000 live births.
o Infant mortality rate: Probability of dying between birth and exactly one
year of age expressed per 1,000 live births.