Cohort ppt

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Cohort ppt

  1. 1. Cohort Study Hari Prasad Kafle Assistant Professor Pokhara University
  2. 2. Cohort Study  Cohort study is undertaken to support the existence of association between suspected cause and disease  A major limitation of cross-sectional surveys and case-control studies is difficulty in determining if exposure or risk factor preceded the disease or outcome.
  3. 3. Cohort Study  Cohort Study: Key Point:  Presence or absence of risk factor is determined before outcome occurs.
  4. 4. What is Cohort ?     Ancient Roman military unit, A band of warriors. Persons banded together. Group of persons with a common statistical characteristic. [Latin] E.g. age, birth date,
  5. 5. What is Cohort ?  Cohort is a group of people who share common characteristic or experience within the defined period of time.  E.g. birth cohort, marriage cohort, Exposure cohort, Synthetic cohort etc.
  6. 6. Cohort studies        longitudinal Prospective studies Forward looking study Incidence study starts with people free of disease assesses exposure at “baseline” assesses disease status at “follow-up”
  7. 7. Indication of a Cohort study     When there is good evidence of exposure and disease. When exposure is rare but incidence of disease is higher among exposed When follow-up is easy, cohort is stable When ample funds are available
  8. 8. C ohort Desig n Study population free of disease Factor present Factor absent present time Study begins here disease no disease disease no disease future
  9. 9. Cohort Studies Disease Population People without disease Exposed No disease Not exposed Disease No disease
  10. 10. Frame work of Cohort studies Disease Status Total Exposure Status Yes No a+b c+d N Yes a No b c d a+c b+d Study cohort Comparison cohort
  11. 11. Types of Cohort Study    Prospective cohort study Retrospective (historical) cohort study Combination of Retrospective and Prospective cohort study.
  12. 12. Prospective cohort study Exposure Study starts Disease occurrence time Study starts Exposure Disease occurrence time
  13. 13. Retrospective cohort studies Exposure Disease occurrence Study starts time
  14. 14. Cohort Studies
  15. 15. General consideration while selection of cohorts     Both the cohorts are free of the disease. Both the groups should equally susceptible to disease Both the groups should be comparable Diagnostic and eligibility criteria for the disease should be defined well in advance.
  16. 16. Elements of cohort study 1. 2. 3. 4. 5. Selection of study subjects Obtaining data on exposure Selection of comparison group Follow up Analysis
  17. 17. Selection of study subjects  General population    Whole population in an area A representative sample Special group of population  Select group   occupation group / professional group Exposure groups  Person having exposure to some physical, chemical or biological agent  e.g. X-ray exposure to radiologists
  18. 18. Obtaining data on exposure  Personal interviews / mailed questionnaire  Reviews of records   Dose of drug, radiation, type of surgery etc Medical examination or special test  Blood pressure, serum cholesterol
  19. 19. Obtaining data on exposure  Environmental survey   Water, Air, Sanitation status etc. By obtaining the data of exposure we can classify cohorts as   Exposed and non exposed and By degree exposure we can sub classify cohorts
  20. 20. Selection of comparison group  Internal comparison    Only one cohort involved in study Sub classified and internal comparison done External comparison   More than one cohort in the study for the purpose of comparison e.g. Cohort of radiologist compared with ophthalmologists
  21. 21. Selection of comparison group  Comparison with general population rates   If no comparison group is available we can compare the rates of study cohort with general population. Cancer rate of uranium miners with cancer in general population
  22. 22. Follow-up  To obtain data about outcome to be determined (morbidity or death)      Mailed questionnaire, telephone calls, personal interviews Periodic medical examination Reviewing records Surveillance of death records Follow up is the most critical part of the study
  23. 23. Follow-up  Some loss to follow up is inevitable due to death change of address, migration, change of occupation.  Loss to follow-up is one of the draw-back of the cohort study.
  24. 24. ANALYSIS  Calculation of incidence rates among exposed and non exposed groups  Estimation of risk
  25. 25. Incidence rates of outcome Disease Status Yes Exposure Status Yes No a c No b d a+c b+d Total a+b c+d N Study cohort Comparison cohort
  26. 26. Incidence rate   Incidence among exposed = a a+b Incidence among non-exposed = c c+d
  27. 27. Estimation of risk  Relative Risk incidence of disease among exposed RR = ______________________________ Incidence of disease among non-exposed a/a+b = _________ c/c+d
  28. 28. Estimation of Risk  Attributable Risk Incidence of disease among exposed – incidence of disease among non exposed AR = _______________________________ Incidence of disease among exposed
  29. 29. Attributable Risk a/a+b – c/c+d AR = _______________ a/a+b
  30. 30. Smoking Lung cancer Total YES NO YES 70 6930 7000 NO 3 2997 3000 73 9927 10000 Find out RR and AR for above data
  31. 31. Incidence Rates  Incidence of lung cancer among smokers 70/7000 = 10 per 1000  Incidence of lung cancer among non-smokers 3/3000 = 1 per thousand
  32. 32. Relative Risk and Attributable Risk RR = 10 / 1 = 10 (lung cancer is 10 times more common among smokers than non smokers) AR = 10 – 1 / 10 X 100 = 90 % (90% of the cases of lung cancer among smokers are attributed to their habit of smoking)
  33. 33. Strengths of Cohort studies      Can establish population-based incidence Accurate relative risk (risk ratio) estimation Can examine rare exposures (asbestos > lung cancer) can establish cause - effect minimizes selection and information bias
  34. 34. Strengths of Cohort studies • Can be used where randomization is not possible • Magnitude of a risk factor’s effect can be quantified • More than one disease related to single exposure • Multiple outcomes can be studied (smoking > lung cancer, COPD, larynx cancer)
  35. 35. Weaknesses of Cohort studies       Often requires large sample long time to complete expensive Ethical issues Non response, migration and loss-to-followup biases Unexpected environmental changes may influence the association
  36. 36. THANK YOU
  37. 37. Examples Presentation of cohort data:
  38. 38. 1. Population at risk Does HIV infection increase risk of developing TB among a population of drug users? Population Cases (follow up 2 years) HIV + HIV - 215 289 Source: Selwyn et al., New York, 1989 8 1 EPIET (www)
  39. 39. Does HIV infection increase risk of developing TB among drug users? Exposure Population (f/u 2 years) Cases Incidence (%) Relative Risk HIV + 215 8 3.7 11 HIV - 298 1 0.3 EPIET (www)
  40. 40. 2. Person-years at risk Tobacco smoking and lung cancer, England & Wales, 1951 Person-years Cases 102,600 133 Smoke Do not smoke 42,800 3
  41. 41. Various exposure levels
  42. 42. Cohort study: Tobacco smoking and lung cancer, England & Wales, 1951

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