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

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

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

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

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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