2.Epidemiology 1

EPIDEMIOLOGY

Dr. Vijayalaxmi GurungDr. Vijayalaxmi Gurung

EPIDEMIOLOGY

Epidemiology is the basic science of preventive Epidemiology is the basic science of preventive and social medicine.and social medicine.

It cover not only study of disease distribution It cover not only study of disease distribution and causation but also health and health and causation but also health and health related events occurring in human population.related events occurring in human population.

Modern epidemiology has entered the most Modern epidemiology has entered the most exciting phase of its evaluationexciting phase of its evaluation

By identifying risk factors of chronic disease, By identifying risk factors of chronic disease, evaluating treatment modalities and health evaluating treatment modalities and health services, it has provided new opportunities services, it has provided new opportunities for prevention, treatment, planning and for prevention, treatment, planning and improving the effectiveness and efficiency of improving the effectiveness and efficiency of health services.health services.

Epidemiology is derived from the word Epidemiology is derived from the word “epidemic”, which is very old word dating “epidemic”, which is very old word dating back to the 3th Century B.C. back to the 3th Century B.C.

Epidemiology means:Epidemiology means: Epi= amongEpi= among Demos= peopleDemos= people Logos= studyLogos= study

Epidemiology

EPI DEMO LOGOS

Among People, population,man The Study of

The study of anything that happens to people

“That which befalls man”

DEFINITION

There appears to be almost as many There appears to be almost as many definition of epidemiology as there are definition of epidemiology as there are authors who have written on the subject, authors who have written on the subject, ranging from Hippocrates to those of the ranging from Hippocrates to those of the present day.present day.

A short list is given below:A short list is given below:

1.1. That branch of medical science which treats That branch of medical science which treats of epidemics – Parkin, 1873of epidemics – Parkin, 1873

2.2. The science of the mass phenomena of The science of the mass phenomena of infectious diseases- Frost, 1927 infectious diseases- Frost, 1927

3.3. The study of disease, any disease, as a mass The study of disease, any disease, as a mass phenomenon- Greenwood, 1934 phenomenon- Greenwood, 1934

4.4. The study of the distribution and The study of the distribution and determinants of disease frequency in man- determinants of disease frequency in man- macMohan,1960macMohan,1960

Definition of Epidemiology* “ The STUDY of the

DISTRIBUTION and DETERMINANTS of HEALTH-RELATED STATES in specified POPULATIONS, and the application of this study to CONTROL of health problems."

*Last, J.M. 1988. A Dictionary of Epidemiology, 2nd ed.

Although there is no single definition to which all Although there is no single definition to which all epidemiologist subscribe, three components are epidemiologist subscribe, three components are common to most of them.common to most of them.

1.1. Studies of disease frequencyStudies of disease frequency

2.2. Study of the distributionStudy of the distribution

3.3. Study of determinantsStudy of determinants

Clinician Epidemiologist

Patient’s diagnostician Investigations Diagnosis

Therapy

Cure

Community’s diagnostician

Investigations Predict trend

Control

Prevention

AIMS OF EPIDEMIOLOGY

According to the International Epidemiological According to the International Epidemiological Association (IEA), epidemiology has three main aims:Association (IEA), epidemiology has three main aims:

a.a. To describe the distribution and magnitude of health To describe the distribution and magnitude of health and disease problems in human populationsand disease problems in human populations

b.b. To identify etiological factors in the pathogenesis of To identify etiological factors in the pathogenesis of disease; anddisease; and

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

The ultimate aim of epidemiology is to lead to The ultimate aim of epidemiology is to lead to effective action:effective action:

a.a. To eliminate or reduce the health problem or To eliminate or reduce the health problem or its consequences; andits consequences; and

b.b. To promote the health and well-being of To promote the health and well-being of society as a whole.society as a whole.

Kinds of Epidemiology

DescriptiveDescriptive

AnalyticAnalytic

ExperimentalExperimental

Further studies to determine the validity of a hypothesis concerning the occurrence of disease.

Deliberate manipulation of the cause is predictably followed by an alteration in the effect not due to chance

Study of the occurrence and distribution of disease

Methods of epidemiological studies Observational studyObservational study

Descriptive study Descriptive study Cross-sectional studyCross-sectional study Longitudinal studyLongitudinal study

Analytical studyAnalytical study• Case control studyCase control study• Cohort studyCohort study

Experimental studyExperimental study RCTRCT NRCTNRCT

Descriptive Epidemiology Study of the occurrence and

distribution of disease Terms: Time Place Person

What are the three categories of descriptive epidemiologic clues?

□ □ Person:Person: WhoWho is getting sick? is getting sick? □ □ Place:Place: WhereWhere is the sickness is the sickness

occurring?occurring? □ □ Time:Time: WhenWhen is the sickness is the sickness

occurring? occurring?

PPT = person, place, timePPT = person, place, time

Time Secular

Periodic

Seasonal

Epidemic

PersonAge HobbiesSex PetsOccupation TravelImmunization status Personal HabitsUnderlying disease StressMedication Family unitNutritional status SchoolSocioeconomic factors GeneticsCrowding Religion

Steps of Descriptive Epidemiology

Defining the populationDefining the population

Defining the diseaseDefining the disease

Describe the diseaseDescribe the disease

Measurement of diseaseMeasurement of disease

Comparing with known indicesComparing with known indices

Formulation the hypothesisFormulation the hypothesis

Incidence

Incidence rate is a measure of the Incidence rate is a measure of the probability of the event among probability of the event among persons at risk.persons at risk.

Incidence Rates

Population denominator:Population denominator:

IR = IR = Number of new cases during time Number of new cases during time period X Kperiod X K

specified population specified population at riskat risk

Prevalence

Prevalence: Existing cases in a Prevalence: Existing cases in a specified population during a specified specified population during a specified time period (both new and ongoing time period (both new and ongoing cases)cases)

Prevalence is a measure of burden of Prevalence is a measure of burden of disease or health problem in a disease or health problem in a populationpopulation

Prevalence

Prevalence: The number of existing cases in the Prevalence: The number of existing cases in the population during a given time period.population during a given time period.

PR PR ==Number of existing cases during time Number of existing cases during time periodperiod

population at same point in timepopulation at same point in time

Prevalence rates are often expressed as a Prevalence rates are often expressed as a percentage.percentage.

iv) iv) As a surrogate for longitudinal descriptive As a surrogate for longitudinal descriptive studies : say, for studies : say, for weight charting of children weight charting of children from birth till 5 years age, on a monthly basis, from birth till 5 years age, on a monthly basis, ideally, we should collect a group of (say 1000) ideally, we should collect a group of (say 1000) children born recently, and follow them up for children born recently, and follow them up for next 5 years, recording their weight every next 5 years, recording their weight every month. month.

This is the ideal, but often a difficult way This is the ideal, but often a difficult way (longitudinal descriptive study).(longitudinal descriptive study).

To overcome the problems of follow up, what we To overcome the problems of follow up, what we can do is that we can take a “cross sectional” can do is that we can take a “cross sectional” sample of say, 100 children each of ages 1, 2, 3 sample of say, 100 children each of ages 1, 2, 3 and so on, till 60 months and weigh them; the and so on, till 60 months and weigh them; the average weights could be used for making a average weights could be used for making a growth chart as would have been done by a growth chart as would have been done by a longitudinal study. longitudinal study.

Excellent and easy though it may look, the Excellent and easy though it may look, the method has a flaw. method has a flaw.

Cross Sectional Descriptive Studies

Such descriptive studies are done on a Such descriptive studies are done on a sample of the total population and may be sample of the total population and may be community based or hospital based. community based or hospital based.

They are mainly directed to work out the :They are mainly directed to work out the : i) “i) “Prevalence” of a factor of interest, e.g. Prevalence” of a factor of interest, e.g.

prevalence of prevalence of leprosy among general leprosy among general population, i.e. community based, or population, i.e. community based, or prevalence of nosocomial prevalence of nosocomial Pseudomonas Pseudomonas infection in infection in renal units, i.e. hospital based.renal units, i.e. hospital based.

ii) “ii) “Mean” of a factor of interest (e.g. Hb% Mean” of a factor of interest (e.g. Hb% level in ladies in level in ladies in the community; or serum the community; or serum protein levels among patients admitted with protein levels among patients admitted with open pulmonary TB).open pulmonary TB).

iii) iii) Description of a “Pattern” (e.g. pattern of Description of a “Pattern” (e.g. pattern of antibiotic antibiotic prescription in an Acute Medical prescription in an Acute Medical Ward; or knowledge, attitudes and practices Ward; or knowledge, attitudes and practices regarding contraceptives in a community).regarding contraceptives in a community).

The children whom we take up in such a study The children whom we take up in such a study at different age groups are the ones who have at different age groups are the ones who have survived to this age. survived to this age.

It is quite possible that malnourished (under It is quite possible that malnourished (under weight) children might have been dying weight) children might have been dying progressively more, and not reaching the next progressively more, and not reaching the next month of age. month of age.

If this were true, a chart based on cross If this were true, a chart based on cross sectional study would show higher mean values sectional study would show higher mean values as compared to a (more correct) chart based as compared to a (more correct) chart based on longitudinal descriptive study.on longitudinal descriptive study.

Longitudinal Descriptive Studies

In contrast to a cross sectional descriptive In contrast to a cross sectional descriptive study, a longitudinal descriptive study follows study, a longitudinal descriptive study follows up a single group of subjects over a defined up a single group of subjects over a defined period of time. period of time.

These studies are more scientific than cross These studies are more scientific than cross sectional ones but at the same time more sectional ones but at the same time more costly and time consuming. costly and time consuming.

These studies are undertaken with the following These studies are undertaken with the following general objectives :general objectives :

i) To see the i) To see the incidence of a disease (e.g. the incidence incidence of a disease (e.g. the incidence of of poliomyelitis among children in the community or poliomyelitis among children in the community or the incidence of acute glomerulonephritis among the incidence of acute glomerulonephritis among children admitted in hospital, with acute sore throat).children admitted in hospital, with acute sore throat).

ii) To describe the ‘ii) To describe the ‘natural history of a disease’ (e.g. natural history of a disease’ (e.g. the clinical the clinical progression of cases of AIDS in hospital; or progression of cases of AIDS in hospital; or the clinical progression of cases of viral hepatitis in a the clinical progression of cases of viral hepatitis in a community).community).

iii) To describe a health related iii) To describe a health related natural phenomena (e.g. natural phenomena (e.g. the the examples of weight charting of children given earlier).examples of weight charting of children given earlier).

iv) To study the ‘iv) To study the ‘trend’ of a disease (e.g. to see whether trend’ of a disease (e.g. to see whether the the incidence of TB is rising or falling in a community).incidence of TB is rising or falling in a community).

v) To study the ‘v) To study the ‘trend of a health - related phenomena trend of a health - related phenomena (e.g. (e.g. to see whether the blood pressure of children from to see whether the blood pressure of children from well - to - do families rises progressively; or whether the well - to - do families rises progressively; or whether the level of knowledge regarding prevention of AIDS is rising, level of knowledge regarding prevention of AIDS is rising, falling or remaining stationary).falling or remaining stationary).

The basic difference between a cross sectional The basic difference between a cross sectional and a longitudinal descriptive study can be and a longitudinal descriptive study can be appreciated by the essential difference that in a appreciated by the essential difference that in a cross sectional study the researcher examines cross sectional study the researcher examines every subject only once; while in a longitudinal every subject only once; while in a longitudinal study each subject is examined study each subject is examined at least twice. at least twice.

For this reason, a cross – sectional For this reason, a cross – sectional study gives us study gives us the “prevalence” while a longitudinal study gives the “prevalence” while a longitudinal study gives us the “incidence”.us the “incidence”.

Use of descriptive epidemiology

To provides data- magnitude of diseasesTo provides data- magnitude of diseases

Formulation of etiological hypothesisFormulation of etiological hypothesis

Data for planning, organizing and evaluating Data for planning, organizing and evaluating preventive and curative servicespreventive and curative services

Help to conduct the researchHelp to conduct the research

Analytical epidemiology

Second type of epidemiological studiesSecond type of epidemiological studies

Helps to test the hypothesisHelps to test the hypothesis

Individuals are evaluated in analytical studiesIndividuals are evaluated in analytical studies

Always two groups- study and controlAlways two groups- study and control

Uses a comparison group to establish an association Uses a comparison group to establish an association between cause and effectbetween cause and effect

Use of analytical epidemiology

Incidence can be calculatedIncidence can be calculated

Estimation of odd ratio and relative riskEstimation of odd ratio and relative risk

Case –control study

Kind of analytical studyKind of analytical study

It is a retrospective studyIt is a retrospective study

First approach to test causal hypothesisFirst approach to test causal hypothesis

Definition

A case control study is defined as an A case control study is defined as an epidemiological epidemiological approach in which the approach in which the researcher starts by picking up ‘cases’ who researcher starts by picking up ‘cases’ who have already developed a particular disease have already developed a particular disease or ‘outcome’ of interest, and a comparison or ‘outcome’ of interest, and a comparison group (controls) of subjects who, except for group (controls) of subjects who, except for the fact that they have not developed the the fact that they have not developed the particular disease, are otherwise similar to particular disease, are otherwise similar to the cases. the cases.

Having assembled the two groups, the Having assembled the two groups, the investigator finds out the presence (or the investigator finds out the presence (or the history) of the particular ‘exposure’ which she history) of the particular ‘exposure’ which she thinks is a risk factor and compares the two thinks is a risk factor and compares the two groups (cases and controls) as regards the groups (cases and controls) as regards the presence (or history) of exposure.presence (or history) of exposure.

3 Distinct Features

Both exposure and outcome have occurred Both exposure and outcome have occurred before start the studybefore start the study

Proceeds backward from effect to causeProceeds backward from effect to cause

Use the control group to refute or support the Use the control group to refute or support the hypothesishypothesis

Case-control study design

ExposureExposure DiseaseDisease ObserverObserver

??

Choose groups with and without disease, look Choose groups with and without disease, look back at what different exposures they may back at what different exposures they may have hadhave had

Case control studies

Attempt to make inference from existing Attempt to make inference from existing observations (retrospective)observations (retrospective)

Compares patients with outcome/disease with Compares patients with outcome/disease with those without and attempts to identify factors those without and attempts to identify factors that influenced that outcome (or caused that that influenced that outcome (or caused that disease)disease)

Important conceptImportant concept: : start with the result start with the result (disease) and work backwards for the cause(disease) and work backwards for the cause

Steps of case control

Selection of cases and controlSelection of cases and control

MatchingMatching

Measurement of exposure, and Measurement of exposure, and

Analysis and interpretationAnalysis and interpretation

Selection of cases

Definition of casesDefinition of casesDiagnostic criteriaDiagnostic criteriaEligibility criteriaEligibility criteria

Sources of casesSources of casesHospital Hospital General populationGeneral population

Diagnostic Criteria

The diagnostic criteria of the disease The diagnostic criteria of the disease and the stage of the disease to be and the stage of the disease to be included in the study.included in the study.

Ex-Breast of cancer Ex-Breast of cancer Histologically the cases should be of Histologically the cases should be of

same group.same group. The diagnostic criteria should not be The diagnostic criteria should not be

changed till the study is over.changed till the study is over.

Eligibility Criteria

Only the newly diagnosed cases within Only the newly diagnosed cases within the specific period of time are eligible the specific period of time are eligible than the old cases.than the old cases.

Sources of cases

HospitalsHospitals: The cases may be drawn from : The cases may be drawn from a single hospital or a network of a single hospital or a network of hospitals, admitted during the specific hospitals, admitted during the specific period of time.period of time.

The entire case series or a random The entire case series or a random sample of it is selected for the study.sample of it is selected for the study.

Advantage - convenientAdvantage - convenient

General Population: General Population: In a population In a population based case control study, all the cases based case control study, all the cases of the study disease occurring within a of the study disease occurring within a defined geographic area during a defined geographic area during a specific period of time are ascertained.specific period of time are ascertained.

EX: The disease registry or the hospital EX: The disease registry or the hospital networknetwork

Selection of control

Free from the diseaseFree from the disease Must be similar to the cases as possible Must be similar to the cases as possible

except for the absence of the disease under except for the absence of the disease under

the study. the study. Selection of the cases is difficult if the Selection of the cases is difficult if the

disease under investigation occurs in a disease under investigation occurs in a subclinical form.subclinical form.

Sources of controlSources of control Hospital controlHospital control RelativesRelatives NeighborhoodNeighborhood General populationGeneral population

Matching

Frequency matching: Selecting the controls Frequency matching: Selecting the controls in such a manner, so that the proportion of in such a manner, so that the proportion of controls with the certain characteristics is controls with the certain characteristics is identical to the proportion of cases with the identical to the proportion of cases with the same characteristics. same characteristics.

Paired matching: For each case selected for Paired matching: For each case selected for the study ,a control is selected ,who is the study ,a control is selected ,who is similar to the case in terms of specific similar to the case in terms of specific characteristicscharacteristics

Measurement of exposure

By questionnaireBy questionnaire InterviewsInterviews Hospital recordsHospital records Employment recordsEmployment records

Analysis

Exposure rates:Exposure rates: The exposure rate among casesThe exposure rate among cases The exposure rate among the controlsThe exposure rate among the controls

Odds ratio:Odds ratio:

Aim: To assess the relationship between Aim: To assess the relationship between the disease and its possible exposuresthe disease and its possible exposures

Odds ratio

The probability that a particular event The probability that a particular event will occur divided by the probability will occur divided by the probability that a event will not occur.that a event will not occur.

It is also termed as a cross product It is also termed as a cross product ratio.ratio.

Strengths of case-control design

Best study when have rare disease or Best study when have rare disease or outcome outcome

Relatively quick and inexpensiveRelatively quick and inexpensive Low costLow cost Relatively efficientRelatively efficient Feasible Feasible Several exposuresSeveral exposures Data are ready made.Data are ready made.

Cont..

No ethical problems No ethical problems Conclusion can be drawn quickly by the Conclusion can be drawn quickly by the

nested case control study.nested case control study.

Weaknesses (potential biases)

Difficult to select the appropriate controlsDifficult to select the appropriate controls Do not establish the risk(only odds ratio)Do not establish the risk(only odds ratio) More chance of bias.More chance of bias. Reliance on recall or records to determine Reliance on recall or records to determine

the exposures.the exposures. Less analytical compares to cohort studyLess analytical compares to cohort study

Selection (confounding) bias: controls must Selection (confounding) bias: controls must be as similar to cases as possible be as similar to cases as possible

Representativeness bias: cases should be Representativeness bias: cases should be “typical”“typical”

Recall bias: cases may be able to remember Recall bias: cases may be able to remember events better because of its significance or events better because of its significance or may be prompted to remember by may be prompted to remember by investigatorsinvestigators

Ways to combat weakness

Matching: Matching: for each case, find a control for each case, find a control that looks just like him/her in all other that looks just like him/her in all other possible ways except for the disease possible ways except for the disease (same age, race, economic class, etc.)(same age, race, economic class, etc.)

Blinding: Blinding: individual assessing exposures individual assessing exposures should be blinded to whether the should be blinded to whether the person is a case or controlperson is a case or control

Nested case control study

It is type of a case control study that It is type of a case control study that draws its cases and the controls from a draws its cases and the controls from a cohort population that has been cohort population that has been followed for a time period.followed for a time period.

A nested case control study depends on A nested case control study depends on the pre-existence of a cohort.the pre-existence of a cohort.

Nested case control study

The investigator identifies the cases of The investigator identifies the cases of disease that occurs in the cohort during disease that occurs in the cohort during the follow up period as well as the the follow up period as well as the disease free individuals within the disease free individuals within the cohort to serve as controls.cohort to serve as controls.