hlsc 4613 principles of epidemiology instructor: ches jones, phd university of arkansas

HLSC 4613HLSC 4613Principles of EpidemiologyPrinciples of Epidemiology

Instructor: Ches Jones, PhDInstructor: Ches Jones, PhD

University of ArkansasUniversity of Arkansas

ContentsContents

Unit One-Introduction and DefinitionsUnit One-Introduction and Definitions Unit Two-Rates and MeasurementsUnit Two-Rates and Measurements Unit Three-Descriptive EpidemiologyUnit Three-Descriptive Epidemiology Unit Four-Analytic EpidemiologyUnit Four-Analytic Epidemiology Unit Five-Screening and SurveillanceUnit Five-Screening and Surveillance

Unit OneUnit One

Introduction and DefinitionsIntroduction and Definitions

Epidemiology-DefinitionEpidemiology-Definition

Branch of medicine dealing with a Branch of medicine dealing with a combination of knowledge and research combination of knowledge and research methods concerned with the methods concerned with the distributiondistribution and and determinantsdeterminants of health and illness of health and illness in in populationspopulations, and with contributors to , and with contributors to health and control of health problems.health and control of health problems.

Main Components of EpiMain Components of Epi

1)1) An analytic, descriptive component An analytic, descriptive component termed termed classical epidemiologyclassical epidemiology, and, and

2)2) A diagnosis, management of illness, A diagnosis, management of illness, and critical review of literature termed and critical review of literature termed clinical epidemiology.clinical epidemiology.

Evolution of Modern Evolution of Modern EpidemiologyEpidemiology

3 Eras3 Eras MiasmaMiasma (Sanitary statistics) (Sanitary statistics)

– Disease due to bad air.Disease due to bad air.– Prior to 1850 Prior to 1850

Infectious DiseaseInfectious Disease (Germ theory) (Germ theory)– 1850-19301850-1930

Chronic DiseaseChronic Disease (Black box) (Black box)– 1930-present1930-present

Infectious Disease (Acute)Infectious Disease (Acute)

Cause Cause Diagnosis DiagnosisTreatment Treatment Severity Severity

Disease of short durationDisease of short duration

Affects mainly the youngAffects mainly the young

Chronic DiseaseChronic Disease

Cause Cause Diagnosis Diagnosis Treatment (cure) Treatment (cure)

Disease of long induction periodDisease of long induction period

Time allows multiple causes to developTime allows multiple causes to develop

Affects mainly the oldAffects mainly the old

Public Health Public Health ApproachApproach

Implementation

How to do it?

Intervention

Evaluation

What works?

Risk Factor

Identification

What’s the cause?

Surveillance

What’s the problem?

Problem Response

3 Levels of Prevention3 Levels of Prevention

PrimaryPrimary- prevention of the development - prevention of the development of diseaseof disease

SecondarySecondary- early detection and - early detection and treatment of diseasetreatment of disease

TertiaryTertiary-rehabilitation and/or restoration -rehabilitation and/or restoration of effective functioning after treatment of of effective functioning after treatment of diseasedisease

Epidemiologic SurveillanceEpidemiologic Surveillance

DefinitionDefinition

The ongoing process and systematic The ongoing process and systematic collection, analysis, and interpretation of collection, analysis, and interpretation of health data in the process of describing health data in the process of describing and investigating the health status of a and investigating the health status of a population.population.

Epidemiological SurveillanceEpidemiological Surveillance

Two types:Two types:PassivePassive-Disease frequency data collected-Disease frequency data collectedPeriodically. Current results not availablePeriodically. Current results not available

ActiveActive-Disease status is updated -Disease status is updated constantly. Usually as the result of an constantly. Usually as the result of an outbreak or other identified epidemic. Is outbreak or other identified epidemic. Is more costly than passive surveillance.more costly than passive surveillance.

Current Uses of EpidemiologyCurrent Uses of Epidemiology

Identifying the etiology and cause of a Identifying the etiology and cause of a new epidemic or syndrome.new epidemic or syndrome.

ExamplesExamples::– Carpal Tunnel SyndromeCarpal Tunnel Syndrome– Toxic Shock SyndromeToxic Shock Syndrome– Post Traumatic Stress SyndromePost Traumatic Stress Syndrome


Investigating the risk associated with a Investigating the risk associated with a harmful exposureharmful exposure

ExamplesExamples: Health risks associated with:: Health risks associated with:– Radon exposureRadon exposure– LeadLead– Environmental tobacco smokeEnvironmental tobacco smoke– DioxinDioxin


Results from a study Results from a study showing survival showing survival rates following rates following segmental and total segmental and total mastectomies.mastectomies.

• DetermineDetermine if a treatment is effective if a treatment is effective.

70

80

90

100

0 1 2 3 4 5

Total Mastectomy Segmental Mastectomy

Years after Surgery


Study and identify health service Study and identify health service utilization needs and trends.utilization needs and trends.

ExamplesExamples::– Effect of health insurance coverage on Effect of health insurance coverage on

health services used by poor and near-health services used by poor and near-poor populations.poor populations.

– Impact of youth violence on emergency Impact of youth violence on emergency room services and utilizationroom services and utilization


To provide rationalization and justification To provide rationalization and justification for health policy planning.for health policy planning.

ExamplesExamples::– Smoking bansSmoking bans– Gun-control bansGun-control bans– Drunk-driving lawsDrunk-driving laws– Hazardous waste regulationsHazardous waste regulations

Aims of EpidemiologyAims of Epidemiology

1)1) Study occurrence, distribution, and Study occurrence, distribution, and progression of diseases and to describe the progression of diseases and to describe the health status of a population.health status of a population.

2)2) Provide data that will contribute to the Provide data that will contribute to the understanding of the etiology of health and understanding of the etiology of health and diseasedisease

3)3) Promote utilization of epidemiological Promote utilization of epidemiological concepts to the management of health concepts to the management of health services.services.

Types of Epi Strategies UsedTypes of Epi Strategies Used

1)1) DescriptiveDescriptive

2)2) AnalyticAnalytic (retrospective (case-control), (retrospective (case-control), prospective (longitudinal or cohort), prospective (longitudinal or cohort), and cross-sectional)and cross-sectional)

3)3) ExperimentalExperimental (cause and effect) (cause and effect)

Limitations of EpidemiologyLimitations of Epidemiology

Difficult to assess risk from Difficult to assess risk from epidemiology data because:epidemiology data because:

1) Research studies on humans are 1) Research studies on humans are sometimes unethical, expensive, and sometimes unethical, expensive, and difficult to obtain.difficult to obtain.

2) Chronic disease situations often finds 2) Chronic disease situations often finds very low risk.very low risk.

Limitations of EpidemiologyLimitations of Epidemiology(Continued)(Continued)

3) The number of persons with the 3) The number of persons with the disease or exposure is very small.disease or exposure is very small.

4) Latency period between exposure and 4) Latency period between exposure and disease status are sometimes many disease status are sometimes many years apart.years apart.

5) Humans may be exposed to multiple 5) Humans may be exposed to multiple chemical, biological, and physical chemical, biological, and physical hazards.hazards.

Epidemiological ModelsEpidemiological Models

– Traditional ModelTraditional Model

– Health Field ConceptHealth Field Concept

– Other ModelsOther Models

Traditional ModelTraditional Model

Environment

HostAgentAgent

Health Field ConceptHealth Field Concept

Biology/Heredity

EnvironmentLifestyle

Health Care System


LifestyleLifestyle– LeisureLeisure– Consumption patternsConsumption patterns– Employment/occupational risksEmployment/occupational risks

EnvironmentEnvironment– PhysicalPhysical– Social PsychologicalSocial Psychological


Human BiologyHuman Biology– Genetic Inheritance maturation and agingGenetic Inheritance maturation and aging

Medical Care SystemMedical Care System– PreventivePreventive– RestorativeRestorative– CurativeCurative

Use of HFC in EpiUse of HFC in Epi

1)1) Selection of diseases that are of high Selection of diseases that are of high risk and contribute to mortality and risk and contribute to mortality and morbidity.morbidity.

2)2) Allocate resources proportionally to Allocate resources proportionally to disease occurrences.disease occurrences.

3)3) Allocate total health expenditures to Allocate total health expenditures to the four elements of the epidemiology the four elements of the epidemiology model.model.

Web of CausationWeb of Causation

Shows multiple factorsShows multiple factors Antecedents of risk factorsAntecedents of risk factors TimeTime Illustrates complication of disease Illustrates complication of disease

etiologyetiology Identifies intervention pointsIdentifies intervention points

Poor diet

Weight Loss

Job

Stress

High School

Addiction

Smoking

Child

Snacks

High Sugar

Diet

Family Job

No time No Equipment

Lack of Exercise

Diabetes

Concept of RiskConcept of Risk

With multiple causes and chronic With multiple causes and chronic diseases, epidemiologists like to refer to diseases, epidemiologists like to refer to the concept of causality based on the the concept of causality based on the odds (risks, chances) of the occurrence odds (risks, chances) of the occurrence of disease or health status as of disease or health status as associated with the occurrence of a associated with the occurrence of a specific exposure (risk/protective specific exposure (risk/protective factor).factor).

Criteria for determining Criteria for determining causalitycausality

(more applicable to single cause/single effect)(more applicable to single cause/single effect) Temporal relationshipTemporal relationship: a causes b, then a : a causes b, then a

comes firstcomes first SpecificitySpecificity: a cause leads to a single effect: a cause leads to a single effect Strength or intensityStrength or intensity (strong relationship (strong relationship

between findings)between findings) ConsistencyConsistency (same association is found study (same association is found study

after study)after study) CoherenceCoherence (does it make sense?) (does it make sense?)

Criteria for a Risk factorCriteria for a Risk factor

1)1) Risk increases with increased Risk increases with increased exposureexposure

2)2) Time sequenceTime sequenceRisk FactorRisk Factor DiseaseDisease

3)3) Limited or no error involvedLimited or no error involved

Chronic Disease Risk FactorsChronic Disease Risk Factors

Unit TwoUnit Two

Epidemiological MeasurementEpidemiological Measurement


Mortality RatesMortality Rates

Morbidity RatesMorbidity Rates


Where to get data?Where to get data?– Mortality/Vital StatisticsMortality/Vital Statistics– Morbidity/Hospital/Clinic RecordsMorbidity/Hospital/Clinic Records– Health Assessments/Behavior SurveysHealth Assessments/Behavior Surveys– Surveillance SystemsSurveillance Systems

Measures of MortalityMeasures of Mortality

Crude Mortality RateCrude Mortality Rate Infant Mortality RateInfant Mortality Rate Specific Mortality RateSpecific Mortality Rate

(age, sex, race, and cause)(age, sex, race, and cause)

Case Fatality RateCase Fatality Rate Proportionate Mortality Ration (PMR)Proportionate Mortality Ration (PMR)

Epidemiological Epidemiological MeasurementsMeasurements

General FormulaGeneral Formula

Number of eventsNumber of events

(cases, deaths, services)(cases, deaths, services)

In a specified time periodIn a specified time period

Population at risk ofPopulation at risk of

experiencing the eventexperiencing the event

X 10n Some base of ten:1,00010,000100,000

Rates and RisksRates and Risks

Ecological FallacyEcological Fallacy (generalizing) (generalizing) Variations in BaseVariations in Base (what base is used) (what base is used) False AssociationFalse Association (rates apply to pop’n) (rates apply to pop’n) Variance of RatesVariance of Rates

(differences based on rates)(differences based on rates)

Reasons to Use Caution When Interpreting Rates and Risks

Crude Mortality RateCrude Mortality Rate

All deaths during a calendar yearAll deaths during a calendar year

Population at mid-yearPopulation at mid-year

X 1,000 = deaths per 1,000X 1,000 = deaths per 1,000

Infant Mortality RateInfant Mortality Rate

Most widely accepted measure for Most widely accepted measure for estimating the health status of a estimating the health status of a

populationpopulation

Number of infant deaths *Number of infant deaths *(less than 1 year of age)(less than 1 year of age) Number of live birthsNumber of live births

*excludes fetal deaths*excludes fetal deaths

X 1,000 (common rate)

Specific Mortality RatesSpecific Mortality Rates

Before the experiences of two Before the experiences of two populations can be compared, account populations can be compared, account must be taken for differences in age, sex, must be taken for differences in age, sex, race, or cause.race, or cause.

Rates are adjusted in order to remove the Rates are adjusted in order to remove the effect of a confounding variable, such as effect of a confounding variable, such as age, sex, or race.age, sex, or race.

Specific Mortality RatesSpecific Mortality Rates

ExamplesExamples

Mortality Rates Specific For:Mortality Rates Specific For:

Age Specific MR: by age groupAge Specific MR: by age group

Gender Specific MR: for males, for Gender Specific MR: for males, for femalesfemales

Race/Ethnic Group: for white, blacks, etc.Race/Ethnic Group: for white, blacks, etc.

Cause Specific-MortalityCause Specific-Mortality

Deaths assigned to the specifiedDeaths assigned to the specified

disease during a calendar yeardisease during a calendar year

Population at mid-yearPopulation at mid-yearX 100,000

=deaths per 100,000 population per year

Case Fatality RatioCase Fatality Ratio

Number of deaths due to the disease in a Number of deaths due to the disease in a specific time period specific time period

number of cases of the disease in the number of cases of the disease in the same time periodsame time period

X 100X 100Express as %

Case Fatality RatioCase Fatality Ratio

This measure represents the probability This measure represents the probability of death among diagnosed cases, or the of death among diagnosed cases, or the killing power of a disease.killing power of a disease.

Proportionate Mortality RatioProportionate Mortality Ratio

Deaths assigned to the disease in Deaths assigned to the disease in a certain yeara certain year

Total deaths in the population in the same Total deaths in the population in the same yearyear

X 100X 100 Express as %

Proportionate Mortality RatioProportionate Mortality Ratio

Used to describe the proportion of the Used to describe the proportion of the overall mortality that is ascribed to a overall mortality that is ascribed to a specific cause.specific cause.

Morbidity RatesMorbidity Rates

Attack RateAttack Rate IncidenceIncidence PrevalencePrevalence Years of Potential Life Lost (YPLL)Years of Potential Life Lost (YPLL)

Attack RateAttack Rate

An incidence rate used to describe the An incidence rate used to describe the occurrence of food borne illnesses, occurrence of food borne illnesses, infectious diseases, and other acute, infectious diseases, and other acute, short time period diseases.short time period diseases.

illill

ill + wellill + wellX 100 (%)

Attack Rate (example)Attack Rate (example)

ill = 10ill = 10

not ill = 3not ill = 3

Total = 13Total = 13

1010

1313X 100 = 76%

Incidence and PrevalenceIncidence and Prevalence

The two main measures of disease frequency The two main measures of disease frequency (morbidity).(morbidity).

IncidenceIncidence = NEW cases of a certain disease = NEW cases of a certain disease

PrevalencePrevalence = ALL cases of a certain disease = ALL cases of a certain disease

Incidence and PrevalenceIncidence and Prevalence

IncidenceIncidence RecoverRecover

DeathDeath

PrevalencePrevalence

PotPot

IncidenceIncidence

Incidence rates are designed to Incidence rates are designed to measure the rate at which people measure the rate at which people without a disease develop the disease without a disease develop the disease during a specific period of time.during a specific period of time.

Number of new cases of a Number of new cases of a disease over a period of disease over a period of

timetime population at risk of the population at risk of the

disease in the time period disease in the time period

Incidence rate

=

Incidence ExampleIncidence Example

Gonorrhea in ArkansasGonorrhea in Arkansas

19871987

19961996

8898 new cases 2,342,699

5027 new cases 2,509,793

= 381/100,000

= 200/100,000


Prevalence rates measure the number of Prevalence rates measure the number of people in a population who have the people in a population who have the disease at a given point of time.disease at a given point of time.

Prevalence ratePrevalence rate = = Total number of cases of a disease at a given time

Total population at a given time


TypesTypes Annual (yearly)Annual (yearly) Lifetime (overall prevalence)Lifetime (overall prevalence) Period (specific period of time) Period (specific period of time) Point (right now!) Point (right now!)

Years of Potential Life LostYears of Potential Life Lost

Indicates how diseases compare in Indicates how diseases compare in reducing life expectancy. Calculated for reducing life expectancy. Calculated for ages up to 65.ages up to 65.

Example:Example: A person killed at the age of 25 A person killed at the age of 25 has lost 40 years of potential life. (25-has lost 40 years of potential life. (25-65=40)65=40)

Years of Potential Life LostYears of Potential Life Lost

Application:Application:– In 1988, an estimated 1,198,887 years of

potential life lost (YPLL) before age 65 were attributed to smoking.

– Source: CDC. Smoking-attributable mortality and years of potential life lost -- United States, 1988. MMWR 1991;40:62-3,69-71.

AIDS in ArkansasAIDS in ArkansasCounty County BentonBenton CarrollCarroll PulaskiPulaski WashWash

Pop’n 105,588 19,505 353,394 142,737Pop’n 105,588 19,505 353,394 142,737

AIDS 8 0 60 17AIDS 8 0 60 179/95-6/969/95-6/96

AIDS 68 23 687 161AIDS 68 23 687 161TotalTotal

Rate AdjustmentRate Adjustment(Standardized Rates)(Standardized Rates)

Adjustment for differences in populationAdjustment for differences in population

Composition (age, gender, ethnicity, etc.)Composition (age, gender, ethnicity, etc.)

-Direct Adjustment-Direct Adjustment

-Indirect Adjustment-Indirect Adjustment

Direct Method of AdjustmentDirect Method of Adjustment

Application of population composition Application of population composition specific rates to determine the expected specific rates to determine the expected number of events in a standard number of events in a standard population.population.

Uses Standard PopulationsUses Standard Populations

Indirect Method of AdjustmentIndirect Method of Adjustment

Standard rates applied to populations Standard rates applied to populations being compared in order to calculate the being compared in order to calculate the expected number of events, and the expected number of events, and the compared with the observed number of compared with the observed number of events.events.

Uses Standard RatesUses Standard Rates

StandardizedStandardizedMortality/Morbidity Ratio (SMR)Mortality/Morbidity Ratio (SMR)

A rate used for comparing the A rate used for comparing the standardized mortality rates.standardized mortality rates.

Observed DeathsObserved Deaths

Expected DeathsExpected Deaths

Unit ThreeUnit Three

Descriptive EpidemiologyDescriptive Epidemiology

Descriptive and AnalyticDescriptive and AnalyticEpidemiologyEpidemiology

Descriptive EpidemiologyDescriptive Epidemiology-- amount and amount and distribution of disease within a distribution of disease within a population by population by person, place, and timeperson, place, and time

Analytic EpidemiologyAnalytic Epidemiology--more focused more focused study on the determinants of disease or study on the determinants of disease or reason for relatively high or low reason for relatively high or low frequency in specific groups.frequency in specific groups.

Ask these questions:Ask these questions:

Who Who (Person) – D(Person) – D WhatWhat (Type of Disease, illness, (Type of Disease, illness,

disability)-Ddisability)-D When When (Time) –D(Time) –D Where Where (Place) – D (Place) – D How How (Etiology or cause of event) – A (Etiology or cause of event) – A

D= DescriptiveD= Descriptive A= AnalyticA= Analytic

Case DefinitionCase Definition

Standard criteria used to assess Standard criteria used to assess whether a person has a particular whether a person has a particular disease or health condition. Ensures disease or health condition. Ensures that every case is diagnosed using the that every case is diagnosed using the same criteria. Comparisons with time, same criteria. Comparisons with time, place, and populations can be place, and populations can be conducted.conducted.

Foodborne Illness Outbreak Foodborne Illness Outbreak Case DefinitionCase Definition

An incident in which two or more An incident in which two or more persons experience a similar illness persons experience a similar illness after ingestion of a common food, and after ingestion of a common food, and epidemiologic analysis implicates the epidemiologic analysis implicates the food as the source of the illnessfood as the source of the illness

DescriptiveDescriptive

PersonPerson

3 main characteristics:3 main characteristics:– AgeAge– GenderGender– Ethnic Ethnic

AgeAge

AgeAge- is the most important determinant - is the most important determinant among the person variables. Mortality among the person variables. Mortality and morbidity rates of conditions show and morbidity rates of conditions show some relation to age.some relation to age.

Infectious diseaseInfectious disease-younger-younger Chronic diseaseChronic disease- older- older

GenderGender

Mortality- higher among malesMortality- higher among males

Morbidity- higher among femalesMorbidity- higher among females

GenderGenderMortality-Mortality- linked with inheritance, hormonal linked with inheritance, hormonal

balance, environment, or habit patternbalance, environment, or habit pattern

MorbidityMorbidity- women have higher rates of - women have higher rates of illness and more physician contacts than illness and more physician contacts than men. Possible reasons:men. Possible reasons:

1)Women seek medical care more freely and 1)Women seek medical care more freely and perhaps at an earlier stage of disease and,perhaps at an earlier stage of disease and,

2) The same disease will tend to have a less 2) The same disease will tend to have a less lethal dose in women than in menlethal dose in women than in men

Ethnic GroupEthnic Group

Classifying people by ethnic group is Classifying people by ethnic group is difficult but important in field of difficult but important in field of epidemiology. Why:epidemiology. Why:

1.) Many diseases differ in frequency, 1.) Many diseases differ in frequency, severity, or both in different racial severity, or both in different racial groups, and,groups, and,

2.) Statistics by race are helpful for 2.) Statistics by race are helpful for identifying health problems.identifying health problems.

Other Person VariablesOther Person Variables

Social ClassSocial Class OccupationOccupation Marital StatusMarital Status Family VariablesFamily Variables Family SizeFamily Size Birth OrderBirth Order Personality traitsPersonality traits

Maternal AgeMaternal Age Parental DeprivationParental Deprivation Blood TypeBlood Type Environmental Environmental

ExposureExposure

Place considerationsPlace considerations

Frequency of disease can be related to Frequency of disease can be related to place of occurrence by:place of occurrence by:

Natural Boundaries (more useful)Natural Boundaries (more useful)

(such as river, deserts, mountains)(such as river, deserts, mountains)

Political Subdivisions (more convenient)Political Subdivisions (more convenient)

Place considerationsPlace considerations

Mapping environmental factorsMapping environmental factors

Urban-Rural differencesUrban-Rural differences

International comparisonsInternational comparisons

Time considerationsTime considerations

3 major time measurements:3 major time measurements: Secular trends (long-term variations)Secular trends (long-term variations)

Cyclic (recurrent alterations in the Cyclic (recurrent alterations in the frequency of disease)frequency of disease)

Short-term fluctuationsShort-term fluctuations

Unit FourUnit Four

Analytic Study DesignsAnalytic Study Designs

Criteria to Evaluate StudyCriteria to Evaluate Study

1.) Study size—was it large enough1.) Study size—was it large enough

2.) How were subjects selected?2.) How were subjects selected?

3.) Bias prone?3.) Bias prone?

4.) Confounding prone?4.) Confounding prone?

5.) Adequate analysis?5.) Adequate analysis?

6.) Were limitations discussed?6.) Were limitations discussed?

Study Design DefinitionsStudy Design Definitions

ConfoundingConfounding- illusory associated between two - illusory associated between two variables.variables.

Association caused by 3Association caused by 3rdrd factor, “confounder” factor, “confounder”

Example: link between Example: link between coffeecoffee and and colon cancercolon cancer may actually be caused by may actually be caused by smokingsmoking..


BiasBias- Subjects chosen for study are - Subjects chosen for study are unrepresentive of the population. Types unrepresentive of the population. Types of bias include: (over 57 types)of bias include: (over 57 types)– Healthy Worker biasHealthy Worker bias– InformationInformation– VolunteerVolunteer– RecallRecall– ResearcherResearcher

Control for ConfoundingControl for Confounding

PreventionPrevention RandomizationRandomization MatchingMatching RestrictionRestriction

AnalysisAnalysis StratificationStratification Multivariate techniquesMultivariate techniques

Reducing BiasReducing Bias

Case DefinitionCase Definition High Participation RatesHigh Participation Rates Ensure RepresentationEnsure Representation Use Standardized FormsUse Standardized Forms Training of Research PersonnelTraining of Research Personnel Blind Participants and ResearchersBlind Participants and Researchers


ChanceChance- making assumptions and - making assumptions and inferences of the measure of disease inferences of the measure of disease frequency concerning the experience of frequency concerning the experience of a population based on an evaluation of a population based on an evaluation of only a sample. Because of chance only a sample. Because of chance variation, for any two sample in a variation, for any two sample in a population to be identical is highly population to be identical is highly unlikely.unlikely.

Chance-example via checkerboardChance-example via checkerboard

Eliminate Chance FindingsEliminate Chance Findings

P-valueP-value Confidence IntervalsConfidence Intervals Reduce ErrorsReduce Errors

Causation is not measured by Causation is not measured by the P-valuethe P-value

P-value only reflects that results are a P-value only reflects that results are a consequence of chance (random error).consequence of chance (random error).

Not:Not:– Result of bias- (systematic error)Result of bias- (systematic error)– Attributable to confoundingAttributable to confounding– Study is reflecting causal relationshipStudy is reflecting causal relationship– Study design is correctStudy design is correct

Types of Study DesignsTypes of Study Designs

Case Report or Case SeriesCase Report or Case Series Descriptive (Population-based)Descriptive (Population-based) Analytic (Individual-based)Analytic (Individual-based) Follow-Up (Cohort)Follow-Up (Cohort) Case-ControlCase-Control

Follow-Up StudiesFollow-Up Studies(Cohort)(Cohort)

RetrospectiveRetrospective

ProspectiveProspective


Investigates the association between a Investigates the association between a disease and past exposure to a risk disease and past exposure to a risk factor among a cohort.factor among a cohort.


PASTPAST PRESENTPRESENT

Look for pastexposure in population

Select cohort


StrengthsStrengths

1.) Less expensive1.) Less expensive

2.) Faster to do study2.) Faster to do study

LimitationsLimitations

1.) Impossible to control for confounding 1.) Impossible to control for confounding factorsfactors

2.) Bias prone2.) Bias prone


Study starts with a group (cohort) of Study starts with a group (cohort) of people who are free of disease, but who people who are free of disease, but who vary according to exposure to probable vary according to exposure to probable disease factor.disease factor.


PresentPresent FutureFuture

Select cohort andClassify as toExposure to factor

Follow-up tosee frequency withwhich disease develops


StrengthsStrengths

1.) Temporal sequence is clear1.) Temporal sequence is clear

2.) Bias and confounding are relatively 2.) Bias and confounding are relatively easy to controleasy to control

3.) Absolute measure of occurrence are 3.) Absolute measure of occurrence are available (incidence, mortality, etc.)available (incidence, mortality, etc.)

4.) Provides information on many factors4.) Provides information on many factors



1.) Very expensive and time consuming1.) Very expensive and time consuming

2.) May not provide significant findings 2.) May not provide significant findings until after 5-10 yearsuntil after 5-10 years

3.) Inappropriate for rare diseases3.) Inappropriate for rare diseases

4.) Problems with following up on subjects4.) Problems with following up on subjects

5.) Extremely inefficient5.) Extremely inefficient

Case Control StudyCase Control Study

People diagnosed as having a disease People diagnosed as having a disease (cases) are compared with persons who (cases) are compared with persons who do not have the disease (controls) with do not have the disease (controls) with relation to various risk factors.relation to various risk factors.

Case Control StudyCase Control Study

PASTPAST PRESENTPRESENT

Look for pastexposure to factor in casesand controls

Select individualswith the

disease(cases)

Select individualswithout the

disease (controls)

Case-Control StudyCase-Control Study

Dominate form of epidemiologic study Dominate form of epidemiologic study (>80%)(>80%)

Difficult but rewarding design to useDifficult but rewarding design to use Case-control studies have been used in Case-control studies have been used in

other areas besides causation-other areas besides causation-preventive services and health services preventive services and health services researchresearch


StrengthsStrengths1)1) Appropriate for rare diseasesAppropriate for rare diseases2)2) Appropriate for disease with long induction Appropriate for disease with long induction

time.time.3)3) Economical and done rapidlyEconomical and done rapidly4)4) Allow evaluation of multiple hypothesesAllow evaluation of multiple hypotheses5)5) Extremely efficientExtremely efficient6)6) Large amount of information on small Large amount of information on small

amount of subjectsamount of subjects



1)1) People don’t understand it (abused)People don’t understand it (abused)

2)2) Study is poor when exposure of Study is poor when exposure of interests is rareinterests is rare

3)3) Only relative measures are availableOnly relative measures are available

4)4) Bias proneBias prone


Design questionsDesign questions

1)1) Where to get cases?Where to get cases?• Population based (expensive)Population based (expensive)• Selected PopulationSelected Population

2)2) Where to get controls?Where to get controls?• General population (ideal but unrealistic)General population (ideal but unrealistic)• Hospital controlsHospital controls

Measure of RiskMeasure of Risk

Absolute RiskAbsolute Risk Relative RiskRelative Risk Attributable RiskAttributable Risk

Caution with RisksCaution with Risks

1.) All those exposed to the disease factor 1.) All those exposed to the disease factor will not develop the disease or illness will not develop the disease or illness but just have a probability of doing so.but just have a probability of doing so.

2.) Some people not exposed to disease 2.) Some people not exposed to disease factor will develop the disease.factor will develop the disease.

Absolute RiskAbsolute Risk

Synonymous with incidence and means Synonymous with incidence and means the rate of occurrence of the disease.the rate of occurrence of the disease.

Relative Risk and Attributable Relative Risk and Attributable RiskRisk

Epidemiologic measures of the Epidemiologic measures of the association between exposure to a association between exposure to a particular factor and risk of a certain particular factor and risk of a certain outcome.outcome.

Relative RatioRelative Ratio(Odds Ratio)(Odds Ratio)

Incidence rate among exposedIncidence rate among exposed

Incidence rate among non-exposedIncidence rate among non-exposed

Attributable RiskAttributable Risk

I (exposed) – I (non-exposed)I (exposed) – I (non-exposed)

Case-Control AnalysisCase-Control Analysis

Disease StatusDisease Status

Exposure

Status

Yes

No

CA-Yes CO-No

a b

c d

N1 N0

M1

M0

Exposure RatesExposure Rates

Case exposure = Case exposure =

Control exposure =Control exposure =

a

N1

b

N0

Odds Ratio (RR)Odds Ratio (RR)

Odds Ratio = (a x d)Odds Ratio = (a x d)

Among people who (risk factor), the Among people who (risk factor), the incidence of (disease) is (OR) greater or incidence of (disease) is (OR) greater or lower than those who don’t (risk factor).lower than those who don’t (risk factor).

(b X c)(b X c)

Attributable Incident RateAttributable Incident Rate

AIAIE E %= (OR-1)/OR%= (OR-1)/OR

Among people who (risk factor), % of Among people who (risk factor), % of (disease) is attributable to (risk factor).(disease) is attributable to (risk factor).

AIAITT %=(AI %=(AIE E %) (CA%) (CAEE))

If nobody (risk factor), I of (disease) would If nobody (risk factor), I of (disease) would go down by % in the population.go down by % in the population.

Confidence IntervalsConfidence Intervals

Confidence intervals are calculations of Confidence intervals are calculations of the best estimate of the OR. the best estimate of the OR. Researchers are stating that they are Researchers are stating that they are (%) confident that their true range is (%) confident that their true range is between the lower and upper limits of between the lower and upper limits of the confidence interval.the confidence interval.

Unit FiveUnit Five

Screening and SurveillanceScreening and Surveillance

ScreeningScreening

PurposePurpose

To identify people who have an To identify people who have an enhanced probability of receiving a enhanced probability of receiving a disease and have no signs or disease and have no signs or symptoms of disease. A screening test symptoms of disease. A screening test is not intended to be diagnostic.is not intended to be diagnostic.

ScreeningScreening

Characteristics of a good screening program:Characteristics of a good screening program: Targeted at appropriate disease Targeted at appropriate disease Uses a good testUses a good test Has good compliance from targeted Has good compliance from targeted

populationpopulation Follow-up on those tested positive. Assist Follow-up on those tested positive. Assist

them in accessing medical care servicesthem in accessing medical care services

ScreeningScreening

Problems with screeningProblems with screening

1)1) Creates anxiety in peopleCreates anxiety in people

2)2) False sense of reassuranceFalse sense of reassurance

3)3) Produces morbidity through test itselfProduces morbidity through test itself(screening devices and (screening devices and

equipment)equipment)

4)4) Excess morbidityExcess morbidity

No ScreeningNo Screening

BB 2020 4040 5050 55556060

Exposureperiod

Cellsexfoliate

Symptomsdiagnosis

Cancerbegins

Death

Age

ScreeningScreening

BB 2020 4040 5050 55556060

Exposureperiod

Cellsexfoliate

Symptomsdiagnosis

Cancerbegins

Death

Age

ScreeningDetection

ScreeningScreening

Current SituationCurrent Situation

Diagnosis

Self-Referral

Surveillance

Care for ChronicDisease

Recovery

ScreeningScreening

Projection for the FutureProjection for the Future

Diagnosis

Self-Referral

Surveillance

Care for ChronicDisease

Recovery

Three Phases of DiseaseThree Phases of Disease

Pre-Clinical Phase (PCP)-Pre-Clinical Phase (PCP)- begins when begins when cancer begins.cancer begins.

Pre-clinical PhasePre-clinical Phase ends at symptom ends at symptom diagnosisdiagnosis

Detectable Pre-Clinical Phase (DPCP)Detectable Pre-Clinical Phase (DPCP) begins at first possible detection of cancer.begins at first possible detection of cancer.

Detectable Pre-Clinical PhaseDetectable Pre-Clinical Phase ends when ends when symptoms appearsymptoms appear

Clinical PhaseClinical Phase

Age

30

55

45

55

ScreeningScreening

Characteristics of disease that makes it Characteristics of disease that makes it suitable for screening:suitable for screening:

1)1) Serious diseaseSerious disease2)2) Early therapy better than late therapyEarly therapy better than late therapy3)3) The The detectable pre-clinical phasedetectable pre-clinical phase is is

highhigh4)4) There is treatment available for There is treatment available for

diseasedisease

Screening AnalysisScreening Analysis

True DiagnosisTrue Diagnosis

Test ResultsTest Results DiseasedDiseased Not Not Diseased Diseased

PositivePositive a a b b a+ba+b

NegativeNegative c c d c+d d c+d

a+ca+c b+d a+b+c+d b+d a+b+c+d

Measures of a Screening TestMeasures of a Screening Test

True Positive Rate, True Positive Rate, SensitivitySensitivity – a/(a+c) – a/(a+c)Capacity of a test to give a positive finding Capacity of a test to give a positive finding

when the person tested truly has the when the person tested truly has the disease.disease.

True Negative Rate, True Negative Rate, SpecificitySpecificity- d/(b+d)- d/(b+d)Capacity of a test to give a negative finding Capacity of a test to give a negative finding

when the person tested is truly free of when the person tested is truly free of disease.disease.

Measures of Screening TestMeasures of Screening Test

False Negative Rate- c/(a+c)False Negative Rate- c/(a+c)

Percent measure of a test to give a negative Percent measure of a test to give a negative finding when the person tested truly has the finding when the person tested truly has the disease.disease.

False Positive Rate-b/(b+d)False Positive Rate-b/(b+d)

Percent measure of a test to give a positive Percent measure of a test to give a positive finding when the person tested does not have finding when the person tested does not have the disease.the disease.

Screening ExampleScreening Example

True DiagnosisTrue Diagnosis

Test ResultsTest Results DiseasedDiseased Not Not Diseased Diseased

PositivePositive 40 40 100 100 140 140

NegativeNegative 10 10 1,000 1,010 1,000 1,010

5050 1,100 1,100 1,150 1,150

Screening ExampleScreening Example

SensitivitySensitivity (a/a+c) = 80% (a/a+c) = 80%

False Negative Rate (c/a+c) = 20%False Negative Rate (c/a+c) = 20%

SpecificitySpecificity (d/b+d) = 91% (d/b+d) = 91%

False Positive Rate (b/b+d) = 9%False Positive Rate (b/b+d) = 9%

Evaluating Screening ProgramEvaluating Screening Program

Three primary methodsThree primary methods

1.) Process measures1.) Process measuresNumber of people screenedNumber of people screenedNumber of times people were screenedNumber of times people were screenedTotal cost of programTotal cost of programCost per case detectedCost per case detected

Evaluating a Screening Evaluating a Screening ProgramProgram

2.) 2.) Special process measureSpecial process measurePredicted Value PositivePredicted Value Positive (PVP) (PVP) is useful in is useful in

measuring the proportion of positive tests that measuring the proportion of positive tests that are truly positive.are truly positive.

PVP= a/ (a+b)PVP= a/ (a+b)PVP= 40/ 140 = 29%PVP= 40/ 140 = 29%

A high PV signifies a satisfactory test, but alone A high PV signifies a satisfactory test, but alone it does not provide any information on the it does not provide any information on the tests validitytests validity


2.) 2.) Special process measureSpecial process measurePredicted Value Negative (PVN)Predicted Value Negative (PVN) is useful in is useful in

measuring the proportion of negative tests measuring the proportion of negative tests that are truly negative.that are truly negative.

PVP = d/ (c+d)PVP = d/ (c+d)PVP= 1000/1010 = 99%PVP= 1000/1010 = 99%

A high PV signifies a satisfactory test, but alone A high PV signifies a satisfactory test, but alone it does not provide any information on the it does not provide any information on the tests validity.tests validity.


3.) 3.) Outcome measuresOutcome measures

• Mortality of screened diseaseMortality of screened disease• Case Fatality Rate of screened diseaseCase Fatality Rate of screened disease

Problems with Screening Problems with Screening EvaluationEvaluation

Lead Time BiasLead Time Bias-- belief that screening belief that screening program has given more years of life to program has given more years of life to individual who was positively screened individual who was positively screened for disease.for disease.

Length Time BiasLength Time Bias-- belief that screen belief that screen detected cases have a better prognosis detected cases have a better prognosis than symptoms-detected cases.than symptoms-detected cases.

Patient Self-Selection BiasPatient Self-Selection Bias

Epidemiologic SurveillanceEpidemiologic Surveillance

DefinitionDefinition

The ongoing process and systematic The ongoing process and systematic collection, analysis and interpretation of collection, analysis and interpretation of health data in the process of describing health data in the process of describing and investigating the health status of a and investigating the health status of a population.population.

Characteristics of SystemCharacteristics of System

1.1. Public health importance of the health Public health importance of the health event/problemevent/problem

2.2. Describe the surveillance systemDescribe the surveillance system

3.3. Usefulness of systemUsefulness of system

4.4. Evaluate according to 7 attributesEvaluate according to 7 attributes

5.5. Resources used to operate systemResources used to operate system

6.6. Conclusions and RecommendationsConclusions and Recommendations

Public Health ImportancePublic Health Importance

Number of casesNumber of cases IncidenceIncidence PrevalencePrevalence Case fatalityCase fatality Index of severityIndex of severity PreventabilityPreventability Hospital and medical costsHospital and medical costs

Describe SystemDescribe System

Objectives of surveillance systemObjectives of surveillance system Describe the health events (case definition of Describe the health events (case definition of

each health event)each health event) Flow chart of the systemFlow chart of the system Components and operation of systemComponents and operation of system

• PopulationPopulation• Time of data collectionTime of data collection• Information collectedInformation collected• Who provides dataWho provides data• How is information stored, transferredHow is information stored, transferred• How is data analyzed, how oftenHow is data analyzed, how often• How are reports distributed and to whomHow are reports distributed and to whom

Usefulness of SystemUsefulness of System

A surveillance system is useful if it A surveillance system is useful if it contributes to the prevention and control contributes to the prevention and control of a health problem. It may also indicate of a health problem. It may also indicate other health events or problems as other health events or problems as being serious.being serious.

EvaluationEvaluation

Evaluate system based on following attributes:Evaluate system based on following attributes: SimplicitySimplicity-- How simple is the surveillance system How simple is the surveillance system

to useto use FlexibilityFlexibility-- is surveillance system flexible to adapt is surveillance system flexible to adapt

to changing information needs and operating to changing information needs and operating conditionsconditions

AcceptabilityAcceptability-- are health care and public health are health care and public health agencies willing to participate in the surveillance agencies willing to participate in the surveillance systemsystem

SensitivitySensitivity-- how efficient is system in detecting how efficient is system in detecting cases of disease or adverse health conditions.cases of disease or adverse health conditions.

EvaluationEvaluation

Predictive value positivePredictive value positive (PVP)- (PVP)-the the proportion of persons identified as having proportion of persons identified as having cases who actually have the disease.cases who actually have the disease.

RepresentativenessRepresentativeness--does surveillance does surveillance system accurately describe.system accurately describe.

1)1) The occurrence of a health problemThe occurrence of a health problem2)2) Its distribution in the population by place, time, Its distribution in the population by place, time,

and personand person TimelinessTimeliness-the time of reporting cases -the time of reporting cases

within each step of the surveillance system.within each step of the surveillance system.

Resources to Operate SystemResources to Operate System

Personnel requirementsPersonnel requirements Other resourcesOther resources

TravelTravelSuppliesSuppliesEquipment, etc.Equipment, etc.

Conclusions and Conclusions and RecommendationsRecommendations

One of the main purposes of a One of the main purposes of a surveillance system is to provide feedback surveillance system is to provide feedback and information to prevent and control and information to prevent and control disease. After disease has been disease. After disease has been monitored, suggestions and monitored, suggestions and recommendations are provided in order to recommendations are provided in order to facilitate the control of disease and facilitate the control of disease and prevent future outbreaks and occurrences prevent future outbreaks and occurrences of health events.of health events.

Classification SystemsClassification Systems

DefinitionDefinitionAn orderly arrangement of data that serves a An orderly arrangement of data that serves a

specific purpose.specific purpose.Should meet 3 criteria:Should meet 3 criteria:1)1) Classes used must be mutually exclusiveClasses used must be mutually exclusive2)2) It should be exhaustiveIt should be exhaustive3)3) It should have a reasonable number of It should have a reasonable number of

classes and a reasonable frequency of classes and a reasonable frequency of cases in each class.cases in each class.

Examples of Classification Examples of Classification SystemsSystems

International Classification of DiseaseInternational Classification of Disease E-codesE-codes National Ambulatory Medical Care National Ambulatory Medical Care

SurveySurvey Utilization BehaviorUtilization Behavior Diagnosis-Related Groups (DRG)Diagnosis-Related Groups (DRG)

International Classification of International Classification of Diseases (ICD-9 codes)Diseases (ICD-9 codes)

Primarily used to code mortality and Primarily used to code mortality and morbidity cases to obtain statistical morbidity cases to obtain statistical summaries and analysis. Many other summaries and analysis. Many other classification systems base their classification systems base their surveillance mechanisms on this surveillance mechanisms on this system. Most popular and used system.system. Most popular and used system.

Classes = 17 (main sections)Classes = 17 (main sections)

National Ambulatory Medical National Ambulatory Medical Care SurveyCare Survey

Survey gives information concerning Survey gives information concerning ambulatory patients’ visits to primary ambulatory patients’ visits to primary care physician (PCP). It also provides care physician (PCP). It also provides measure of the magnitude and nature of measure of the magnitude and nature of complaints by those who visit PCP.complaints by those who visit PCP.

Classes = 13 (refers to anatomic site or Classes = 13 (refers to anatomic site or system)system)

National Ambulatory Medical National Ambulatory Medical Care SurveyCare Survey

Example:Example:

Estimating the impact of a national Estimating the impact of a national health insurance plan on health care health insurance plan on health care utilization that is based on pilot plans or utilization that is based on pilot plans or studies.studies.

Classification by Utilization Classification by Utilization BehaviorBehavior

Classification system used to group Classification system used to group diseases into classes most likely to diseases into classes most likely to result in similar medical care usage.result in similar medical care usage.

Classes = 10 (disease and non-disease Classes = 10 (disease and non-disease groups)groups)

Classification by Utilization Classification by Utilization BehaviorBehavior

ExampleExample

Useful in linking medical care usage to Useful in linking medical care usage to health conditions. The types and amount health conditions. The types and amount of medical care services could be of medical care services could be assessed. Can provide information on assessed. Can provide information on the value of various interventions or the value of various interventions or services.services.

Diagnosis-Related Groups Diagnosis-Related Groups (DRGs)(DRGs)

Classification system of hospitalized Classification system of hospitalized cases that is used for reimbursing cases that is used for reimbursing hospitals prospectively on a cont-per-hospitals prospectively on a cont-per-case bases for the care of Medicare case bases for the care of Medicare patients. Based on length of stay (LOS) patients. Based on length of stay (LOS) and severity of illness.and severity of illness.

Classes = 23 major diagnostic categoriesClasses = 23 major diagnostic categories

Diagnosis-Related Groups Diagnosis-Related Groups (DRGs)(DRGs)

Example:Example: The Hospital Efficiency and The Hospital Efficiency and Effectiveness AnalysisEffectiveness Analysis

Identifies health care facilities, or areas Identifies health care facilities, or areas within a facility, which have utilization within a facility, which have utilization habits and/or pricing policies habits and/or pricing policies inconsistent with the local market and inconsistent with the local market and those facilities which have superior or those facilities which have superior or adverse outcomes as compared with adverse outcomes as compared with the nations.the nations.

hlsc 4613 principles of epidemiology instructor: ches jones, phd university of arkansas

Documents

health status

health services

etiology of health

health risks

determinants of health

interpretation of health

control of health problems

health policy planning