Introduction to epidemiology and it's measurements
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Transcript of Introduction to epidemiology and it's measurements
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Introduction to Epidemiology and It’s Measurements
Dr. Rajan Bikram Rayamajhi
Senior Resident
School of Public Health and Community Medicine
B. P. Koirala Institute of Health Sciences 1
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Objectives
To understand:
oDefinition of epidemiology
oScope of epidemiology
oPurpose of scientific method for studying diseases
and health problems
oUses of epidemiology
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oEpi : among
oDemos: People
oLogos: Study
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Epidemiology
Based on two fundamental assumptions:
oFirst, human disease does not occur at random
oSecond, human disease has causal and preventive factors
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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 problems1
o There are three closely interrelated components: distribution, determinants and frequency and it encompass all epidemiological principles and methods.
1Last JM: A Dictionary of Epidemiology, Edition 2. New York, Oxford University Press, 1988.
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oThe measurement of disease frequency involves quantification of the existence or occurrence of disease.
oThe availability of such data is a prerequisite for any systematic investigation of patterns of disease occurrence in human populations
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o The distribution of disease considers such questions as
owho is getting the disease within a populationowhere 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
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oThe determinants of disease is derived from the first two; since knowledge of frequency and distribution of disease is necessary to test an epidemiological hypothesis
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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)
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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.
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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 appropriately.
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
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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
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Basic Measurement in Epidemiology
oMorbidity
oMortality
oDisability
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Tools of Measurement
oRatesoRatiosoProportionsoRelative RiskoOdds Ratio
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Uses of Epidemiology
o To study historically the rise and fall of disease in the population
o Community diagnosiso Planning and evaluationo Evaluation of individual’s risks and chanceso Syndrome identificationo Completing the natural history of diseaseo Searching for cause and risk factor
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Basic Measurement
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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 Time
o Measurement of disease frequency is a prerequisite for any epidemiologic investigation
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oEpidemiology 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.
oTo achieve either of these objectives it is first necessary to measure disease frequency.
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o Simply counting the affected individuals is not enough.
oPopulation 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 slaughterhouse.
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Basic Measurement o Ratioo Proportiono Prevalence Rate o Incidence rateo Case fatality rate o Mortality rates(age specific/cause specific)o Attack rate
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RATIO: A fraction in which the numerator is not part of the denominator.
Ex: Fetal death ratio: Total no. of fetal deaths/total no. of live births
Fetal deaths are not part of live births
PROPORTION: A fraction in which the numerator is part of the denominator.
Ex: Proportional mortality. o Most fractions in epidemiology are proportions.
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RATE: A proportion in which change over time is considered
- But in practice, the term “ rate” is often used interchangeably with ratio without reference to time
Ex: fetal death rate & fetal death ratio, maternal mortality rate & maternal mortality ratio.
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Prevalence vs. Incidence
oPrevalence: frequency of existing caseso Incidence: frequency of new cases
o New cases are called incident cases.o Existing cases are called prevalent cases.
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PREVALENCE RATE:
No. of people with disease at specified time
No. of people in Population at risk at specified time
x 10n
expressed as cases per 1000/ 10,000
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oPoint prevalence: Proportion of a population affected by a disease at a given time.
oPeriod Prevalence: Proportion of individuals in a specified population at risk who have the disease of interest over a specified period of time.
Ex: annual prevalence rate.(When the type of prevalence rate is not
specified it is usually point prevalence, or its closest practical approximation) 25
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Prevalence
Increased by :o longer duration of the diseaseo prolongation of life without cureo increase of new cases( i. e. incidence)o in-migration of caseso out-migration of healthy peopleo in-migration of susceptible peopleo improved diagnostic facilities
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Decreased by:
oShorter disease durationoHigh case fatality from diseaseodecrease in new casesoIn-migration of healthy peopleoOut-migration of casesoImproved cure rate of cases
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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 planning.
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 28
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INCIDENCE RATEIncidence 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
X 1000
Prevalence = Incidence x Duration of disease
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Incidence Rate
o Larger studies: mid-period population
o Also called incidence density or force of morbidity (mortality)
o Expressed as number of new cases per person-time at risk
o Person-time can be person-days, person-months, person years, but more common is per 100 person-years 30
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oFor 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.
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Cumulative Incidence
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
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Attack Rate (AR)
o Cumulative incidence during an outbreako Usually expressed for the entire epidemic period,
from the first to the last caseo 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%
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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 X 100
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Cause specific mortality rate
o No. of deaths from a specific disease
in a population in a given period x 1000 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.
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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 x 100
Total deaths from all causes in that year
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Other Measures
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 year.
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.
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Thank you
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