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Design and Methods of Cohort Studies

Neuroepidemiology Teaching Course: Non-Experimental Neuroepidemiology Chisinau Moldavia September 26 2012. Design and Methods of Cohort Studies. Giancarlo Logroscino MD PhD Department of Neurology and Psychiatry University of Bari. Italy.

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Design and Methods of Cohort Studies

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  1. Neuroepidemiology Teaching Course: Non-Experimental Neuroepidemiology Chisinau Moldavia September 26 2012 Design and Methods of Cohort Studies Giancarlo Logroscino MD PhD Department of Neurology and Psychiatry University of Bari. Italy

  2. Veterans Gain U.S. Benefits for Lou Gehrig’s Disease • E-Mail • Print • Reprints • Save • Share • Linkedin • Digg • Facebook • Mixx • YAHOO! BUZZ • Permalink • By DENISE GRADY • Published: September 23, 2008 • The federal government will provide disability pay, • lifetime health care and death benefits for all veterans with • Lou Gehrig’s disease, the Department of Veterans Affairs said Tuesday, • saying the disease was linked to military service. • Skip to next paragraph • RSS Feed • Get Health News From The New York Times » • All veterans with the illness will be eligible, regardless of when or where they served. • The 10-year cost for death and disability benefits is projected at $505,839,000, • said Tom Pamperin, the deputy director of the compensation and pension service • at the Veterans Affairs Department. That figure does not include health care costs. • The decision is based on studies suggesting that veterans are more likely to develop the disease, • an often fatal nerve disorder, which is also known as amyotrophic lateral sclerosis or A.L.S. September 28, 2008

  3. Some general statements about epidemiology

  4. Study Design: Two Steps • We determine whether there is an association between a factor or a characteristics and the development of a disease • We establish appropriate inferences regarding a possible causal relationship based on the patterns of the association that has been found

  5. Goal of Epidemiology • Quantification of causal relationship between exposure and disease(K. Rothman 1986) • Measurement of the association focuses on quantitative index that characterizes the strength of the association • Quantification of an association is a measure of state of nature

  6. Null Effect

  7. The Natural History of a Disease Gordis L, Epidemiology 2000 W. B. Saunders Company Philadelphia, PA Preclinical Phase Clinical Phase Outcome (A) (P) (S) (M) (D) (T) Biological Onset Pathological Evidence Signs and Symptoms of Disease Medical Care sought Diagnosis Treatment time

  8. Time and the Induction Period of Disease Preclinical Phase Clinical Phase Exposure Signs and Symptoms of Disease Diagnosis Biological Onset time

  9. time

  10. Classification of Studies Descriptive Studies Populations Frequency Distribution by time Place Person • Analytic Studies • Counting exposure and outcomes in individuals • Test causal hypotheses • Uncontrolled assignment • Experimental Studies • Counting exposure and outcomes in individuals • Test causal hypotheses • Controlled assignment RandomizedClinical Trial Population Survey Case-Control Cohort

  11. Define the hypothesis • Define the independent variable (risk factor) • -Exposed cohort • -Unexposed cohort • Define the dependent variable (disease) • Perform measures • -Initial study/baseline assessment • -Follow-up and detection of incident cases • Statistical analysis • -Incidence and relative-risk • -Person-years, survival curves • Interpretation of results (bias) Anatomy of the Cohort Study

  12. COHORT STUDIES • Study design Then follow to see whether Incidence Rates of disease Totals Disease Develop Disease Not develop a a + b First Select a + b c c + d c d c + d Not exposed

  13. COHORT STUDIES • Results of a Hypothetical Cohort Study of Smoking and Coronary Heart Disease(CHD) Incidence per 1000 per year Develop CHD Do Not develop CHD First Select • 2916 3000 28.0 • 87 4913 5000 17.4 Do not smoke cigarettes RR= Ie/ Ine=

  14. Do we need observational studies?

  15. Individuals and Populations

  16. Observational epidemiolgy and Cohort Studies Experimental Study (RCT) Observational Study POPULATION OTHER-THAN RANDOM ALLOCATION (e.g., Self Selection) RANDOM ALLOCATION GROUP A GROUP B GROUP B

  17. Animal Models • Complete Control of Exposure • Random Allocation

  18. Individuals and Populations

  19. Lost in translation: treatment trials in the SOD1 mouse and in human ALS.Benatar M Neurobiology of Disease 2007 Apr;26(1):1-13

  20. Why did the US Government take that decision about Veterans and ALS?

  21. Veterans Gain U.S. Benefits for Lou Gehrig’s Disease • E-Mail • Print • Reprints • Save • Share • Linkedin • Digg • Facebook • Mixx • YAHOO! BUZZ • Permalink • By DENISE GRADY • Published: September 23, 2008 • The federal government will provide disability pay, • lifetime health care and death benefits for all veterans with • Lou Gehrig’s disease, the Department of Veterans Affairs said Tuesday, • saying the disease was linked to military service. • Skip to next paragraph • RSS Feed • Get Health News From The New York Times » • All veterans with the illness will be eligible, regardless of when or where they served. • The 10-year cost for death and disability benefits is projected at $505,839,000, • said Tom Pamperin, the deputy director of the compensation and pension service • at the Veterans Affairs Department. That figure does not include health care costs. • The decision is based on studies suggesting that veterans are more likely to develop the disease, • an often fatal nerve disorder, which is also known as amyotrophic lateral sclerosis or A.L.S. September 28, 2008

  22. Inference from Epidemiological EvidenceSavitz DA Oxford University Press 2003: 8 • Association between exposure and disease among study participants • Causal effect of exposure on disease in study population • Causal effect of exposure on disease in external population • Prevention of disease through elimination or reduction of the exposure • Public Health Impact from elimination or reduction of the exposure

  23. Gulf War Veterans: the ALS occurrence • 2482333 individuals who were on active duty or activate reserve • 696118 deployed to the Gulf region • 516 ALS cases from passive and active ascertainment • 96.2% successfully contacted

  24. DMDC-reported Deployment Status: Occurrence of ALS among Gulf War VeteransHorner et al, Neurology, 2003; 61:742-749

  25. 1.Define the hypothesis 2.Define the independent variable (risk factor) -Exposed cohort -Unexposed cohort 3. Define the dependent variable (disease) 4.Perform measures -Initial study/baseline assessment -Follow-up and detection of incident cases 5.Statistical analysis -Incidence and relative-risk -Person-years, survival curves 6. Interpretation of results (bias) 1.War RF for ALS 2. Deployment in GW Lack of deployment in GW 3.ALS 4.Deployment Status Ascertainment Detection of ALS cases 5 Measurement of RR Anatomy of the Cohort Study

  26. Population Concept of a Cohort Study Bhopal R. Concept of Epidemiology. An Integrated Introduction to the Ideas, Theories, Principles and Methods of Epidemiology. 2002 Oxford University Press, Oxford Time 0/Now Time 1/Future NE E NE ALS NE NE ALS E E NE E E NE ALS E NE NE NE NE ALS E E NE NE E E NE E E NE NE E ALS ALS E CHD Induction Period future now time

  27. Excess of Incidence of ALS in Young Gulf War VeteransHaley, Neurology, 2003; 61:750-756 SMR (Ratio O:E)

  28. ALS and The Gulf War Veterans: a Possible Cluster • Onset of disease in young age groups that have a very low rate in the general population • There is an increasing slope of the epidemic curve: the excess of cases is still to come • The excess in incidence probably triggered by some environmental exposure

  29. ALS mortality by military service and birth cohort Weisskopf et al Neurology: 2005;64:32-37

  30. Relative Risk of ALS by Service During War Periods (1989-1998) Weisskopf et al Neurology: 2005;64:32-37 p for trend <0.004

  31. What is the Cause of ALS excess of cases among Gulf War veterans?

  32. ALS and Military Service:Hints for Causality Exposure precedes the outcome The risk peaks after a time period (induction period) The risk increase with increasing dose of exposure

  33. Advantage and Disadvantages Disdvantages Advantages • Non-respondent/volunteer • bias (selection) • Diagnostic suspicion bias • (measurement) • Attrition • Time and expense (big sample) • Difficult to study uncommon diseases • Study may alter outcome • Ethical problems • Limited and fixed number of hypotheses (risk factors) • Measure relative risk • Less misclassification • Less measurement bias • Changes in exposure and • gradation of exposure • Multiple disease for the same risk factor

  34. Potential Contribution of Study DesignBhopal R, Concepts of Epidemiology, Oxford 2002 Criteria Cross-sectional Case-control Cohort Trial Temporality Sometimes Sometimes Often Usually Strength or Dose-response Always Always Sometimes Often Experimental confirmation Sometimes (repeated studies) Sometimes (natural changes) Seldom Always Yes (risk or preventive factor(s)) Yes (disease) Yes (risk factor(s)) Specificity Sometimes Biological Plausibility Not Directly Not directly Not directly Not directly Consistency Yes Yes Yes Yes

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