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Advanced course in field epidemiology Health Protection Surveillance Centre Ireland

Advanced course in field epidemiology Health Protection Surveillance Centre Ireland. Objectives To strengthen understanding of various epidemiological studies available in field epidemiology. James Stuart & Alain Moren, June 2006. Presentations on methods. Study design Reference group

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Advanced course in field epidemiology Health Protection Surveillance Centre Ireland

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  1. Advanced course in field epidemiologyHealth Protection Surveillance Centre Ireland • Objectives To strengthen understanding of various epidemiological studies available in field epidemiology James Stuart & Alain Moren, June 2006

  2. Presentations on methods • Study design • Reference group • Counfounding and effect modification • Matching • Mesures or impact • Alternative designs • Introduction to multivariable analysis • How to interpret data • How to make an oral scientifi presentation

  3. Case studies • Smoking and lung cancer • Trichinosis • Toxic shock syndrom • Tiramisu • Presentations from participants • One each day • Evening session?

  4. Epidemiological studies Two types Observation Experiment James Stuart & Alain Moren, June 2006

  5. Experiment Exposed Disease occurrence Exposure assigned Not exposed Unethical to perform experiments on people if exposure is harmful

  6. If exposure not harmful Treatment Preventive measure (vaccination) Blinded Doses Time period Risk - effect No bias Randomised Controlled Trial If not possible Left with observation of experiments designed by Nature Cohort studies Case control studies

  7. Cohort studies marching towards outcomes

  8. What is a cohort? • One of 10 divisions of a Roman legion • Group of individuals • sharing same experience • followed up for specified period of time • Examples • birth cohort • cohort of guests at barbecue • occupational cohort of chemical plant workers • EPIET cohort 10

  9. follow-up period

  10. Calculate measure of frequency: • Cumulative incidence • Incidence proportion • Attack rate (outbreak) • Incidence density end offollow-up

  11. Cohort studies • Purpose • Study if an exposure is associated with outcome(s)? • Estimate risk of outcome in exposed and unexposed cohort • Compare risk of outcome in two cohorts • Cohort membership • Being at risk of outcome(s) studied • Being alive and free of outcome at start of follow-up

  12. exposed unexposed Cohort studies

  13. Incidence among exposed Incidence amongunexposed Cohort studies exposed unexposed

  14. Presentation of cohort data: 2x2 table ill not ill 49 49 98 ate ham did not ham 4 6 10

  15. Presentation of cohort data: Population at risk Does HIV infection increase risk of developing TB among a population of drug users? Population Cases (f/u 2 years) HIV + 215 8 HIV - 289 1 Source: Selwyn et al., New York, 1989

  16. Presentation of cohort data: Person-years at risk Tobacco smoking and lung cancer, England & Wales, 1951 Person-years Cases Smoke 102,600 133 Do not smoke 42,800 3 Source: Doll & Hill

  17. Presentation of data: Various exposure levels

  18. Study starts Study starts Disease occurrence Exposure time Prospective cohort study Disease occurrence Exposure time

  19. Study starts Retrospective cohort studies Disease occurrence Exposure time

  20. Recipe: Cohort study • Identify group of • exposed subjects • unexposed subjects • Follow up for disease occurrence • Measure incidence of disease • Compare incidence between exposed and unexposed group

  21. Our objective is to compare: an incidence rate in an exposed population to the rate that would have been observed in the same population, at the same time if it had not been exposed

  22. Principle of case control studies

  23. Source population Exposed Unexposed

  24. Source population Exposed Cases Unexposed

  25. Source population Exposed Sample Cases Unexposed Controls

  26. Source population Cases Exposed Sample Unexposed Controls = Sample of the denominator Representative with regard to exposure Controls

  27. Intuitively if the frequency of exposure is higher among cases than controls then the incidence rate will probably be higher among exposed than non exposed.

  28. Case control study Exposure ? ? Disease Controls Retrospective nature

  29. Distribution of cases and controls according to exposure in a case control study Cases Controls Exposed a b Not exposed c d Total a + c b + d % exposed a/(a+c) b/(b+d)

  30. Distribution of myocardial infarction cases and controls by oral contraceptive use Oral Myocardial contraceptives Infarction Controls Yes 693 320 No 307 680 Total 1000 1000 % exposed 69.3% 32 %

  31. Distribution of myocardial infarction cases and controls by amount of physical activity Physical Myocardial activity Infarction Controls >= 2500 Kcal 190 230 < 2500 Kcal 176 136 Total 366 366 % exposed 51.9% 62.8 %

  32. Volvo factory, Sweden, 3000 employees, Cohort study 200 cases of gastroenteritis Water Cases Controls Consumption YES 150 ? NO 50 ? Total 200 200

  33. Two types of case control studies • Exploratory New disease New risk factors Several exposures "Fishing expedition" • Analytical Precise a single hypothesis Dose response

  34. Incidence among exposed Incidence amongunexposed Cohort studies exposed unexposed

  35. Ie • Iue Effect measures in cohort studies • Absolute measures • Risk difference (RD) Ie - Iue • Relative measures • Relative risk (RR) • Rate ratio • Risk ratio Ie = incidence in exposed Iue= incidence in unexposed

  36. ill not ill Incidence ate ham 49 49 9850 % did not eat ham 4 6 10 40 % Risk difference 50% - 40% = 10% Relative risk 50% / 40% = 1.25

  37. Does HIV infection increase risk of developing TB among drug users?

  38. Vaccine efficacy (VE) VE = 1 - RR = 1 - 0.28 = 72%

  39. N1 I1 a1 High N2 I2 a2 Medium N3 I3 a3 Low Nne c Unexposed Iue Various exposure levels Exposure level Population Cases Incidence at risk

  40. N1 I1 a1 High N2 I2 a2 Medium N3 I3 a3 Low Nne c Unexposed Iue Various exposure levels Population Cases Incidence RR Exposure level at risk RR1 RR2 RR3 Reference

  41. Cohort study: Tobacco smoking and lung cancer, England & Wales, 1951 Source: Doll & Hill

  42. A cohort study allows to calculate indicators which have a clear, precise meaning. The results are immediately understandable.

  43. Cohort studies • Rate • Rate difference • Rate Ratio (strength of association) Case control studies No calculation of rates Proportion of exposure Any way of estimating Rate ratio ?

  44. Population denominator Cases E a P1 } I1 = a / P1 a/P1 I1/ I0 = ------ c/P0 c P0 E I0 = c /P0 Population sample Cases a I1 = -------- P1/10 } E a P1 /10 a/P1 I1/ I0 = ------ c/P0 c I0 = -------- P0/10 c P0 /10 E

  45. Source population Cases Pop. E a P1 I1 = a / P1 } a/P1 I1/ I0 = ------ c/P0 c P0 E I0 = c /P0 = sample Cases Controls E a b P1 b --- = ---- P0 d c d E

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