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
Current Uses of EpidemiologyCurrent Uses of Epidemiology
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
Current Uses of EpidemiologyCurrent Uses of Epidemiology
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
Current Uses of EpidemiologyCurrent Uses of Epidemiology
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
Current Uses of EpidemiologyCurrent Uses of Epidemiology
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
Health Field ConceptHealth Field Concept
LifestyleLifestyle– LeisureLeisure– Consumption patternsConsumption patterns– Employment/occupational risksEmployment/occupational risks
EnvironmentEnvironment– PhysicalPhysical– Social PsychologicalSocial Psychological
Health Field ConceptHealth Field Concept
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
Epidemiological MeasurementEpidemiological Measurement
Mortality RatesMortality Rates
Morbidity RatesMorbidity Rates
Epidemiological MeasurementEpidemiological Measurement
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
PrevalencePrevalence
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
PrevalencePrevalence
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..
Study Design DefinitionsStudy Design Definitions
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
Study Design DefinitionsStudy Design Definitions
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
RetrospectiveRetrospective
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.
RetrospectiveRetrospective
PASTPAST PRESENTPRESENT
Look for pastexposure in population
Select cohort
RetrospectiveRetrospective
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
ProspectiveProspective
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.
ProspectiveProspective
PresentPresent FutureFuture
Select cohort andClassify as toExposure to factor
Follow-up tosee frequency withwhich disease develops
ProspectiveProspective
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
ProspectiveProspective
LimitationsLimitations
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
Case-Control StudyCase-Control Study
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
Case-Control StudyCase-Control Study
LimitationsLimitations
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
Case-Control StudyCase-Control Study
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
Evaluating a Screening Evaluating a Screening ProgramProgram
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.
Evaluating a Screening Evaluating a Screening ProgramProgram
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.