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![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,](https://image.slidesharecdn.com/cohortppt-140210103410-phpapp01/75/Cohort-ppt-4-2048.jpg)
Uploaded byPokhara University, Pokhara, Nepal
Cohort ppt
- Cohort studies examine the association between an exposure and an outcome by following groups over time and comparing their experience. - This document discusses what a cohort study is, how it differs from other study designs in determining temporal relationships, and provides examples of cohort designs and their analysis. - Key aspects reviewed include prospectively following groups based on exposure status and comparing disease incidence rates and relative risks between exposed and unexposed groups over time.
In this document
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Introduction to cohort study by Hari Prasad Kafle, highlighting the importance of this study design.
Cohort studies establish associations between causes and diseases, focusing on risk factors prior to the outcome.
Cohorts are defined as groups sharing common characteristics, like age or exposure, across time.
Cohort studies are longitudinal, prospective, assessing exposures and disease incidence over time.
Cohorts are indicated when evidence of exposure and disease exists, with stability and funding available.
Framework includes disease assessment pre and post exposure, separating study and comparison cohorts.
Includes prospective, retrospective, and combined cohort studies with focus on exposure timing.
Selecting cohorts involves ensuring comparability and absence of disease at study initiation.
Key elements include selection methods, data acquisition, follow-up processes, and analysis.
Methods include surveys and interviews for exposure data, classifying cohorts as exposed/non-exposed.
Comparison groups can be internal or external to better evaluate exposure's effect on disease.
Follow-up is critical for data collection and outcome assessment; loss to follow-up is a major challenge.
Analysis entails calculating incidence rates among exposed and non-exposed groups.
Relative and attributable risks are calculated to understand exposure impact on disease incidence.
Example showcasing calculating incidence rates and risks for smoking and lung cancer, demonstrating significant findings.
Cohort studies allow establishing incidence rates, relative risk, and examining multiple disease outcomes.
Challenges include large sample requirements, time costs, ethical concerns, and potential biases.
Closing gratitude expressed, reiterating the importance of understanding cohort studies.
Presenting examples of cohort data related to HIV and tuberculosis risk among drug users.
Investigating incidence and relative risk in populations based on HIV status and tuberculosis cases.
An overview of cohort statistics relating tobacco smoking to lung cancer incidence in England & Wales.
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Cohort ppt
- 1. Cohort Study Hari PrasadKafle Assistant Professor Pokhara University
- 2. Cohort Study Cohort studyis 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. Cohort Study Cohort Study: KeyPoint: Presence or absence of risk factor is determined before outcome occurs.
- 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. 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. Cohort studies longitudinal Prospective studies Forwardlooking study Incidence study starts with people free of disease assesses exposure at “baseline” assesses disease status at “follow-up”
- 7. Indication of aCohort 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
- 10. C ohort Design Study population free of disease Factor present Factor absent present time Study begins here disease no disease disease no disease future
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- 12. Frame work ofCohort 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
- 13. Types of CohortStudy Prospective cohort study Retrospective (historical) cohort study Combination of Retrospective and Prospective cohort study.
- 14. Prospective cohort study Exposure Studystarts Disease occurrence time Study starts Exposure Disease occurrence time
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- 17. General consideration while selectionof 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.
- 18. Elements of cohortstudy 1. 2. 3. 4. 5. Selection of study subjects Obtaining data on exposure Selection of comparison group Follow up Analysis
- 19. Selection of studysubjects 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
- 20. Obtaining data onexposure Personal interviews / mailed questionnaire Reviews of records Dose of drug, radiation, type of surgery etc Medical examination or special test Blood pressure, serum cholesterol
- 21. Obtaining data onexposure 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
- 22. Selection of comparisongroup 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
- 23. Selection of comparisongroup 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
- 24. Follow-up To obtain dataabout 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
- 25. Follow-up Some loss tofollow 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.
- 26. ANALYSIS Calculation of incidencerates among exposed and non exposed groups Estimation of risk
- 27. Incidence rates ofoutcome 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
- 28. Incidence rate Incidence amongexposed = a a+b Incidence among non-exposed = c c+d
- 29. Estimation of risk RelativeRisk incidence of disease among exposed RR = ______________________________ Incidence of disease among non-exposed a/a+b = _________ c/c+d
- 30. Estimation of Risk AttributableRisk Incidence of disease among exposed – incidence of disease among non exposed AR = _______________________________ Incidence of disease among exposed
- 31. Attributable Risk a/a+b –c/c+d AR = _______________ a/a+b
- 32. Smoking Lung cancer Total YES NO YES 70 6930 7000 NO 3 2997 3000 73 9927 10000 Find outRR and AR for above data
- 33. Incidence Rates Incidence oflung cancer among smokers 70/7000 = 10 per 1000 Incidence of lung cancer among non-smokers 3/3000 = 1 per thousand
- 34. Relative Risk andAttributable 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)
- 35. Strengths of Cohortstudies 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
- 36. Strengths of Cohortstudies • 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)
- 37. Weaknesses of Cohortstudies 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
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- 40. 1. Population atrisk 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)
- 41. Does HIV infectionincrease 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)
- 42. 2. Person-years atrisk Tobacco smoking and lung cancer, England & Wales, 1951 Person-years Cases 102,600 133 Smoke Do not smoke 42,800 3
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- 44. Cohort study: Tobaccosmoking and lung cancer, England & Wales, 1951