1 / 63

Epidemiological studies

Epidemiological studies. Two types. Observation Experiment. Experiment. Exposed. Disease occurrence. Exposure assigned. Not exposed. Unethical to perform experiments on people if exposure is harmful. If exposure not harmful Treatment Preventive measure (vaccination). Blinded

tiara
Download Presentation

Epidemiological studies

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Epidemiological studies Two types Observation Experiment

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

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

  4. Cohort studies marching towards outcomes

  5. 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 12

  6. follow-up period

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

  8. 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 • Being free of outcome at start of follow-up

  9. exposed unexposed Cohort studies

  10. Incidence among exposed Incidence amongunexposed Cohort studies exposed unexposed

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

  12. 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

  13. 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

  14. Presentation of data: Various exposure levels

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

  16. Study starts Retrospective cohort study Disease occurrence Exposure time

  17. 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

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

  19. Principle of case control studies

  20. Source population Exposed Unexposed

  21. Source population Cases Exposed Unexposed

  22. Source population Cases Exposed Sample Unexposed Controls

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

  24. 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

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

  26. 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)

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

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

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

  30. Two types of case control studies • ExploratoryNew disease New risk factors Several exposures "Fishing expedition" • AnalyticalDefine a single hypothesis Dose response

  31. So, uh, well… case-control studies are just fishing expeditions!

  32. Incidence among exposed Incidence amongunexposed Cohort studies exposed unexposed

  33. 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

  34. 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

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

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

  37. 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

  38. 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

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

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

  41. 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 ?

  42. 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

  43. 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

  44. Source population Cases Pop. E a P1 I1 = a / P1 } c P0 E I0 = c /P0 a/P1a . P0a . d I1 / I0 = ------ = ------- = ----- = c/P0c . P1c . b a / c ------ b / d = sample Cases Controls E Since d/b=P0 / P1 a b P1 b --- = ---- P0 d c d E

  45. Case control study design Cases Controls Odds ratio E a b a ba x d ---- --- = --- ---- c d b x c c d E

More Related