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Basic Ideas and Terminology

Basic Ideas and Terminology. Ettore Beghi Institute for Pharmacological Research Mario Negri, Milano, Italy. EPIDEMIOLOGY. Discipline which studies the frequency and the determinants of a given disease in a well-defined population. PRINCIPAL AIMS OF EPIDEMIOLOGY.

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Basic Ideas and Terminology

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  1. BasicIdeas and Terminology Ettore Beghi InstituteforPharmacologicalResearch Mario Negri, Milano, Italy

  2. EPIDEMIOLOGY Discipline which studies the frequency and the determinants of a given disease in a well-defined population

  3. PRINCIPAL AIMS OF EPIDEMIOLOGY • Calculationof the distributionof a disease in a givenpopulation • Definitionofriskfactors and etiologicalfactors • Developmentofstrategiesfordiseaseprevention • Planning ofhealthassistance

  4. RELEVANT ISSUES IN EPIDEMIOLOGICAL STUDIES • Representativeness of the study population • Sources of cases • Diagnosis (disease definition) • Criteria for the assessment of causality • Criteria for the assessment of disease course and impact of treatments

  5. DESCRIPTIVE (Population Survey) In populations Frequency of disease Distribution of disease - time - place - person ANALYTIC (Case-control & Cohort Studies) In individuals Test casual hypotheses Uncontrolled assignment CLASSIFICATION OF EPIDEMIOLOGICAL STUDIES

  6. SAMPLING AND BIAS Target population Measurements Hypothesis testing Intended sample Actual sample Sampling biases

  7. BIAS “Any systematic error in the design, conduct, or analysis of a study that results in a mistaken estimate of an exposure’s effect on the risk of disease” Schlesselman, 1982

  8. DIAGRAM OF THE IDENTIFICATION OF A DISEASE IN THE GENERAL POPULATION Kurtzke, 1978

  9. SOURCES OF NEUROLOGICAL DISEASES IN EPIDEMIOLOGICAL STUDIES • Hospital records • Ambulatoryrecords • Electrophysiological (EMG) records • Generalpractitioners’ files • Disabilityrecords • Layassociations • Tertiarycenters • Death certificates • Diagnosisrelatedgroups (DRGs) • Diseaseregistries

  10. MEASURES OF DISEASE FREQUENCY • INCIDENCE: Number of individuals in a population that become ill in a stated period of time • CUMULATIVE INCIDENCE: Proportion of a fixed population that becomes ill in a stated period of time • PREVALENCE: Proportion of a population affected by a disease at a given point of time • MORTALITY: Number of individuals in a population died for a disease in a stated period of time

  11. PREVALENCE AND INCIDENCE Prevalence = Incidence x average duration Incidence Migrating in Migrating out Prevalence Death Recovery

  12. DIAGNOSIS • In the presenceof diagnostic markers, the diagnostic process is simplified • In the absenceof diagnostic markers, the diagnosis is based on criteria implying a validation process and consensus among caring physicians

  13. VALIDITY & RELIABILITY OF A DIAGNOSTIC TEST • VALIDITY: capability to identify as positivethose affected by the disease and as negative those not affected by the disease • RELIABILITY: capability to obtain the same results in different occasions (1. Assessment of the same patient at different times; 2. Assessment of the same patient by different investigators)

  14. VALIDITY OR ACCURACY Disease Positive Negative Positive Test Negative Sensitivity = True positives___ a__ PPV = True positives__ a__ Total with dis a+c Total tested pos a+b Specificity = Total negatives__ d NPV = True negatives_ d__ Total without dis b+d Total tested neg c+d

  15. EPILEPTIFORM ABNORMALITIES General Population (n=1000) 955 40 5 3 Sens = 60% Spec = 96% PPV = 7% NPV = 99% Goodin & Aminoff, 1984

  16. EPILEPTIFORM ABNORMALITIES Epilepsy Center (n=1000) 480 260 240 20 Sens = 52% Spec = 96% VPP = 93% VPN = 67% Goodin & Aminoff, 1984

  17. RELIABILITY Observer Instrument Object Interobserver Repeatability or Agreement Intraobserver

  18. RELIABILITY Observer 2 Positive Negative Positive Observer 1 Negative Percent = Positive + negative agreements x 100 = a+d x 100 Agreement All observations N Kappa = Observed % agreement – Expected % agreement 100% - expected % agreement

  19. KAPPA STATISTIC • Parameter quantifying inter-rater agreement adjusting for chance agreement • Its value ranges from 0 (chance agreement) to 1 (perfect agreement) • As measured by kappa, agreement is poor (<0.25), fair (0.25-0.50), good (0.50-0.75), almost perfect (>0.75)

  20. INTER-OBSERVER AGREEMENT ON EEG CONCLUSIONS(Dichotomous Scale)(*) (*)Epileptiform = yes/no Van Donselaaret al, 1992

  21. OBSERVATIONAL CRITERIA FOR CAUSATION • Temporalsequence • Consistencyofassociation • Strengthofassociation • Biologicalgradient • Specificityofassociation • Biologicalplausibility Bradford-Hill, modified

  22. DESIGN OF STUDIES ASSESSING DISEASE ETIOLOGY Schoenberg, 1983

  23. ODDS RATIO (OR) • Is a measureofassociationcloselyrelatedto the relative risk(RR) • Approximates the RR for rare diseases • In the 2 x 2 tableDiseaseExposure Yes No Yes A B No C D Oddsofexposure A/C among the cases and B/D in the controls; the ratioof the oddsofexposureis:OR = (A/C) : (B/D) = AD / BC

  24. RELATIVE RISK (RR) • The relative riskis the ratiobetween the rate (risk) ofdisease in thosewith the exposurefactorand the rate (risk) ofdisease in thosewithout the factor RR = R (exp) / R (nexp)

  25. RELEVANCE OF CAUSAL ASSOCIATION • Relative Risk or OddsRatio - Definite > 10 - Highlyprobable 4-10 - Probable 2.5-3.9 - Possible 1.1-2.4

  26. Considerations When Studying Mortality • Death among people with the condition • Death due to the condition Courtesyof Giancarlo Logroscino

  27. STANDARDIZED MORTALITY RATIO • The standardized mortality ratio or SMR, is a quantity, expressed as either a ratio or percentage quantifying the increase or decrease in mortalityof a study cohort with respect to the general population

  28. WHY TO CALCULATE THE STUDY POWER • A studyshouldbesufficientlylargetoavoidtwoimportantstatisticalerrors: - Assumingthat a differencebetweengroupsisrealwhileitis a chance finding (alphaerror) - Assumingthatthere are no differencesbetweengroupswhen a differenceisactuallypresent (beta error)

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