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Present: Disease Past: Exposure

CHP400: Community Health Program - lI. STUDY DESIGNS. Present: Disease Past: Exposure. Observational / Analytical Studies. Time is Key. Research Methodology. Present: Disease & Exposure. Present: Exposure Future: Disease. nbmmb. Cross - section. Cohort. Cohort Study.

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Present: Disease Past: Exposure

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  1. CHP400: Community Health Program - lI STUDY DESIGNS Present: Disease Past: Exposure Observational / Analytical Studies Time is Key Research Methodology Present:Disease & Exposure Present: Exposure Future: Disease nbmmb Cross - section Cohort Cohort Study Case - control

  2. Content • Definition • Characteristics and types • Design • Advantages and disadvantages • Follow-up • Analysis

  3. Research Methodology STUDY DESIGNS Experimental Observational Animal Experiment Descriptive Analytical Case report Case control Case series Human Intervention Clinical trial Cohort Study Cross section Cohort Ecological

  4. Research Methodology STUDY DESIGNS Observational / AnalyticalStudies Cohort Study Definition: A cohort is a group of people who share a common characteristic or experience Cohort study: An observational, analytic, longitudinal, epidemiologicstudy in which a particular outcome, such as death from a heart attack, is compared in groups of people who are alike in most ways but differ by a certain characteristic, such as smoking ( EXPOSURE )

  5. Research Methodology STUDY DESIGNS Observational / AnalyticalStudies Cohort Study Characteristics : • A “cohort” is a group of people, referred to as “disease-free population” or “population at risk” • A survey is first carried out to exclude prevalent cases from the cohort • We know the exposure status, looking for the disease status • A period of "follow-up“ is specified, for possible new cases' occurrence

  6. Research Methodology STUDY DESIGNS Observational / AnalyticalStudies Cohort Study Types: Two types are recognized: • Prospective(longitudinal): forward in time follow-up study • Retrospective(historical): backward in time study (depends on records: medical / employment). This is the type preferred under occupational settings.

  7. Research Methodology STUDY DESIGNS Observational / AnalyticalStudies Cohort Study Design : Disease Exposed People without disease No disease Population Disease Not exposed No disease

  8. Research Methodology COHORT STUDIESCLINICAL TRIALS STUDY DESIGNS Observational / AnalyticalStudies Cohort Study  EXPOSURE OUTCOME/DISEASE TIME ? YES NO ?

  9. Research Methodology COHORT STUDIESCLINICAL TRIALS STUDY DESIGNS Observational / AnalyticalStudies Cohort Study  EXPOSURE OUTCOME/DISEASE TIME YES ? CASE-CONTROL STUDIES NO ?  EXPOSURE OUTCOME/DISEASE TIME ? YES NO ?

  10. Research Methodology STUDY DESIGNS Observational / AnalyticalStudies Cohort Study INDICATIONS: ?? • 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

  11. Research Methodology Elements of cohort study STUDY DESIGNS Observational / AnalyticalStudies Cohort Study • Selection of study subjects • Obtaining data on exposure • Selection of comparison group • Follow up • Analysis

  12. Research Methodology Selection of study subjects STUDY DESIGNS Observational / AnalyticalStudies Cohort Study • General population • Whole population in an area • A representative sample • Special Exposure groups of population • Occupation/professional groups • e.g. Radiologists exposure to X-ray

  13. Research Methodology STUDY DESIGNS Observational / AnalyticalStudies Cohort Study 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

  14. Research Methodology STUDY DESIGNS Observational / AnalyticalStudies Cohort Study Obtaining data on exposure • Environmental survey • By obtaining the data of exposure we can classify cohorts as • Exposed and non exposed and • By degree exposure we can sub classify cohorts

  15. Research Methodology Selection of comparison group STUDY DESIGNS Observational / AnalyticalStudies Cohort Study • 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

  16. Research Methodology Selection of comparison group STUDY DESIGNS Observational / AnalyticalStudies Cohort Study • 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

  17. Research Methodology STUDY DESIGNS Observational / AnalyticalStudies Cohort Study 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

  18. Research Methodology STUDY DESIGNS Observational / AnalyticalStudies Cohort Study Follow-up • Some loss to follow up is inevitable due to death, change of address, migration, change of occupation. • Follow up is the most critical part of the study

  19. Research Methodology STUDY DESIGNS Observational / AnalyticalStudies Cohort Study Follow-up Techniques : • Periodical medical examinations and mailed questionnaires. • Direct personal interviews or examinations. • Videoconference, neighbors, friends and relatives.

  20. Research Methodology STUDY DESIGNS Observational / AnalyticalStudies Cohort Study Follow-up Techniques : 4. Lost persons can be traced through letters, their relatives or friends. 5. Migrated cohort subjects can also be traced through travel and immigration authorities. 6. Dead persons: local or regional Mortality Registers or Death Certificates.

  21. Research Methodology STUDY DESIGNS Observational / AnalyticalStudies Cohort Study Problems during Follow-up : • Follow-up of a large group. • Limited resources. • Time constraint. • Paucity of trained personnel • Attrition

  22. Research Methodology STUDY DESIGNS Observational / AnalyticalStudies Cohort Study Attrition Reduction: • Obtaining an informed consent. • Recording commitment to continue and cooperate in the study. • Tracing LOST subjects. • Considering Information of lost subjects at the time of analysis • Keeping non-response at a low level to improve the validity.

  23. Research Methodology STUDY DESIGNS Observational / AnalyticalStudies Cohort Study Analysis • Calculation of incidence rates among exposed and non exposed groups • Estimation of risk

  24. Research Methodology Incidence rate STUDY DESIGNS Observational / AnalyticalStudies Cohort Study ANALYSIS Disease Status Yes No Total Study cohort a+b Yes a b Exposure Status Comparison cohort c+d c d No Total a+c b+d N a Incidence among exposed = a+b c Incidence among unexposed = c+d

  25. Research Methodology Estimation of risk STUDY DESIGNS Observational / AnalyticalStudies Cohort Study ANALYSIS Relative Risk (RR) incidence of disease among exposed _____________________________ RR = Incidence of disease among non-exposed a/a+b _________ RR = c/c+d

  26. Research Methodology Estimation of risk STUDY DESIGNS Observational / AnalyticalStudies Cohort Study ANALYSIS Attributable Risk (AR): Incidence of disease among non exposed Incidence of disease among exposed _______________________________ AR = Incidence of disease among exposed a/a+b – c/c+d _______________ = AR a/a+b

  27. Research Methodology STUDY DESIGNS Observational / AnalyticalStudies Cohort Study Calculate RR and AR for above data

  28. Research Methodology STUDY DESIGNS Observational / AnalyticalStudies Cohort Study • Incidence of lung cancer among smokers 70/7000 = 10 per 1000 • Incidence of lung cancer among non-smokers 3/3000 = 1 per 1000 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)

  29. Research Methodology STUDY DESIGNS Observational / AnalyticalStudies Cohort Study The Ideal Cohort : • An ideal cohort should be: • Stable. • Cooperative. • Committed • Well-informed

  30. Research Methodology STUDY DESIGNS Observational / AnalyticalStudies Cohort Study Advantages: • No temporal ambiguity (suggests cause-effect relationship) • Calculation of incidence rates • Suitable for rare exposures • Several outcomes can be studied, after follow-up starts. • Factors associated with selection cannot influence disease status and hence the results.

  31. Research Methodology STUDY DESIGNS Observational / AnalyticalStudies Cohort Study Disadvantages : • Expensive • Time-consuming • May be impractical • Loss to follow-up may affect sample-size

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