Epidemiology basics

An introductory and comprehensive sketch :Epidemiology and research methodology:

Dr. Srabana Misra Bhagabaty, MDDepartment of Preventive Oncology

Dr.B.Borooah Cancer Institute

Headlines• Definitions of epidemiology• Dawn of epidemiology: history• Aims of epidemiology• Components of basic epidemiology• Types of epidemiological studies and methods• Basic measurements in epidemiology• Association and causation

Dr Srabana Misra Bhagabaty

Classically speaking

Epidemiology derived from the word epidemicWhich is a very old word dating back to 3rd centuryBC• Epi = upon• Demos = people• Ology = science• Epidemiology = the science which deals with what falls

upon people…..• Bridge between biomedical, social and behavioral

sciencesDr Srabana Misra Bhagabaty

Old definitions1.THE BRANCH OF MEDICAL SCIENCE WHICH TREATS

EPIDEMICS (Parkin,1873)

2.The science of the mass phenomena of infectious diseases(Wade Hampton Frost ,1927)

3.The study of disease, any disease,as a mass phenomenon (Greenwood,1934)

4.The study of the distribution and determinants of disease frequency in man(Mac Mahon, 1960)


1988: John M .Last definition

“ The study of the distribution and determinants of health related states or events in specified populations and application of this study to the control of health problems”


A Modern Definition

Study of the occurrence and distribution of health-related diseases or events in specified populations, including the study of the determinants influencing such states, and the application of this knowledge to control the health problem (Porta M, Last J, Greenland S. A Dictionary of Epidemiology, 2008)


Father of modern epidemiology>The first cases of cholera in England were reported in1831>Between 1831 and 1854, tens of thousands of people inEngland died of cholera > “Within 76 meters of the spot where Cambridge Street joins

Broad Street there were upwards of 500 fatal attacks of cholera in 10 days

Dr. Snow worked around the clock to track down information from hospital and public records on when the outbreak began and whether the victims drank water from the Broad Street pump.

Ref:1




Cholera, is a Greek word, which means the gutter of the roof. It is caused by bacteria: Vibrio cholerae, which was discovered in 1883 by Robert Koch during a diarrheal outbreak in Egypt.


Who is an epidemiologist ?A professional who strives to study and control the factors that influence the occurrence of disease or health-related conditions and events in specified populations and

societies, has an experience in population thinking and epidemiologic methods, and is knowledgeable about public health and causal inference in health

(Porta M, Last J, Greenland S. A Dictionary of Epidemiology, 2008)


Aims of EpidemiologyAccording to the International Epidemiological

Association(IEA)

> To describe the distribution and magnitude of health and disease problems in human populations

To identify etiological factors (risk Factors) in the pathogenesis of the disease

To provide data essential to the planning implementation and evaluation of services for the prevention, control and treatment of the disease and to the setting up of priorities amongst those services


Ultimately

To eliminate or reduce the health problem or its consequences

To promote the health and wellbeing of the society as a whole


Three Components epidemiology

Main three components

ofepidemiology

Disease distribution

Disease distribution

Diseasefrequency

Diseasedeterminants


Component:1 disease frequency

It is to measure the disease or health situation tounderstand gravity/emergency of the issue

How it is measured: by biostatistical tools forbasic measurements in epidemiology in the formof RATES and RATIOS


Component:2 disease distributionDescriptive epidemiology towards

hypothesis formulationIn terms ofTime: to understand disease fluctuation

patterns in relation to time i.e. short term/periodic /long term

Place in terms of international, national, rural-urban, local variations

Person in terms of age, sex, ethnicity, marietal status, occupation, social status, behaviour, stress and migration


Component:3 Disease determinantsThe real substance of epidemiology

analytical epidemiology

• Tests hypothesis • Understanding determinants of diseases from

more solid proofs• For formulating scientifically sound health

programs, interventions and policies


Types of epidemiological studies and methods

Observational studies a) Descriptive b) Analytic >ecological /correlation >cross sectional/prevalence >case control/ case reference

>cohort/follow up


EXPERIMENTAL /INTERVENTION STUDIES

a) Randomized controlled trials /clinical trials

b) Field trials /community intervention trials

c) Community trials


Basic measurements in epidemiology

• Measurements in epidemiology: The scope is very broad, unlimited and includes the following (ref-4)

1. Mortality2. Morbidity 3. Disability 4. Presence/absence or distribution of the characteristics or

attributes as well as suspected environmental and other factors of the disease

5. Medical needs, available facilities and utilization of these medical or other health related services

6. Demographic variables


BASIC REQUREMENTS IN EPIDEMIOLOGICAL TESTS/ MEASUREMENTS

Is the quality control to reduce

errors


By assuring

1.Validity/ accuracy

2.Reliability/precision/reproducibility

Of the epidemiological measurements and tools

Sensitivity

Specificity


VALIDITY

Validity refers to the closeness with which measured values agree with “ true” values

Two components of validity are

A. SensitivityB. Specificity


Sensitivity

Introduced by yerushalmy (ref:5) in 1940 asstatistical index of diagnostic accuracy

IT IS TRUE POSITIVEExample of true positive : for example if a testfor a disease is 90 percent sensitive that means 90 out of 100 diseased people will show a positivetest result where as only 10 out of 100 disesed people will show a negative result (false negative)

90 percent sensitivity

100 People with the disease

90 0ut of 100 diseased shows a positive test 10 out of 100 diseasedshowed a negative result


Specificity

TRUE NEGATIVE

Ability of a test to identifycorrectly those who do nothave the disease


90 percent specificity

100 People without the disease

90 0ut of 100 non- diseased shows a negative test

10 out of 100 non-diseasedshowed a positive result


Calculation of validity

Test Diagnosis TotalDiseased Not Diseased

Positive A ( true positive) B ( false positive) A + B

Negative C ( false negative) D (true Negaive) C+D

Total A+C B+D A+B+C+D


Sensitivity = true positive (A) -------- ------------------------- ---------X 100 true positive (A)+ false negative(C)

Specificity= true negative (D) ------------------------------------------------X 100 false positive( B) + true negative(D)


Tools of measurement expression of disease magnitude

1. Rates2. Ratios3. Proportion


Before going to the basic tools of measurements…

Numerator : Numerator refers to the number of times an

event has occurred in a population, during a specified time period.

Denominator : Gives meaning to the numerator


RATE

• Measures occurrence of some particular event in a population during a given time period

• The epidemiologist has to choose an appropriate denominator while calculating a rate.

a) Related to the population b) Related to the total events


Related to the population denominators

• Mid-year population• Population at risk• Person time• Person distance• Population sub groups


Related to the total events

Related to the total events in stead of the totalPopulation

Example : accidents rate calculation may have

denominators like per 100/1000/….vehicles than total population as all population may not use vehicle


Elements of RATE• Numerator• Denominator• Time specification• Multiplier

Expression of rate : as per 1000 or some other round figures selectedaccording to convenience and to avoid fractions


Categories of rate

• Crude rates/ unstandardized rates : actually observed rates. Example: birth and death rates.

• Specific rates : Actually observed rates due to specific cause; specific groups; specific time periods

• Standardized rates: these are obtained by direct or indirect methods of standardization


RATIO

• RELATION IN SIZE OF TWO RANDOM QUANTITIES• THE NEMERATOR IS NOT A PART OF THE

DENOMINATOR

PROPORTION A proportion is a ratio which indicates the

relation of a part of the whole. The numerator is always included in the denominator

Measurement of morbidityIncidence

Number of new cases occurring in a definedpopulation during a specified period of time

Number of spells of sickness starting in a defined period---------------------------------------------------------------------------X 100Mean number of persons exposed to risk in that period

Uses of incidence: a) control disease b) for research : etiology, pathogenesis disease distribution efficiency of preventive and therapeutic measures

Prevalence Prevalence: All current (old and new) cases existing at a given point or over aPeriod of time in a given population.Types of prevalence : POINT PREVELENCE Number of all current cases of a specific disease existing at a given point of

time-------------------------------------------------------------------------------------------------------

X100Estimated population at the same point of time

PERIOD PREVELENCE:Number on existing cases during a period----------------------------------------------------------X100Estimated mid-interval population at risk

Uses of prevalence:a) Estimates the magnitude of health/ disease problems in the community b) Identify high risk populationsc) Administrative and planning purposes e.g hospital beds, man powerneeds, rehabilitation facilities etc.

• Relation of incidence to prevalence :• Prevalence= Incidence X Mean Duration • P=IXD

• ASSOCIATION and CAUSATION : concurrence of two variables more often than would be expected by chanceCorrelation determines the degree of associationCorrelation coefficient varies from -1to +1 Correlation may not measure risk.Causation implies correlation but co-relation may not imply causation

ASSOCIATION AND CAUSATION

Types of association:

A) Spurious association:B) Indirect association:C) Direct association:

Criteria for judging causality:

>Temporal association :>Strength of association: >Specificity of association:>Consistency of the association:>Biological plausibility:>Coherence of association:

uses of epidemiology

• History : rise and fall of disease in the population

• Community diagnosis• Planning and evaluation• Evaluation of individuals risks and chances• Syndrome identification• Completing the natural history of disease• Searching for cause and risk factorsDr Srabana Misra Bhagabaty

References

1.Snow J. On the mode of communication of cholera. 2nd ed. In: Snow on Cholera. (Reprint.) New York, New York: Hafner Publishing Co., 1965. Available at http://www.ph.ucla.edu/epi/snow.html.

2. Bumberg, M.S(1966).In:Chronic Diseases and Public Health,Lilienfeld, et al (eds). John Hopkins

3. Park textbook of Preventive and Social Medicine: 21st edition 4.Fox ,J.P etal(1970). Epidemiology : Man and disease, New

York, Macmillan5. WHO(1959) Techn. Rep. Ser.,No. 164


“I keep six honest serving-men (They taught me all I knew);

Their names are What and Why and When

And How and Where and Who”

following the story "Elephant's Child" in "Just So Stories" Poem by Rudyard Kipling:


Thank you Dr Srabana Misra Bhagabaty

Epidemiology basics

Health & Medicine