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Cohort Study
Cohort Study
Cohort Study
Cohort Study
Cohort Study
Cohort Study
Cohort Study
Cohort Study
Cohort Study
Cohort Study
Cohort Study
Cohort Study
Cohort Study
Cohort Study
Cohort Study
Cohort Study
Cohort Study
Cohort Study
Cohort Study
Cohort Study
Cohort Study
Cohort Study
Cohort Study
Cohort Study
Cohort Study
Cohort Study
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Cohort Study

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  • 1. Dr Manish Chandra Prabhakar MGIMS Sewagram
  • 2. Cohort studies  longitudinal  Prospective studies  Forward looking study I  Incidence study  Follow up study  starts with people free of disease  assesses exposure at “baseline”  assesses disease status at “follow-up”
  • 3.  Purpose: - Study of the association between an exposure factor and one or more diseases outcomes - Confirmation of hypothesis (a step further towards causation)
  • 4.  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.  Cohort Study: Key Point:  Presence or absence of risk factor is determined before outcome occurs.
  • 5.  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 sample funds are available
  • 6.  Two groups are studied: - Study group (cohort - Control group Defined according to the presence or the absence of the exposure under investigation
  • 7.  Defined as those who: - Have the exposure factor under the study (working definition of the exposure should be set) - Are free from the disease or outcome under study and, - Are at risk of developing this disease or outcome
  • 8.  Selected to represent all the population of exposed individuals  Source of selection may be; -workplace -Registries - Hospital records - community
  • 9.  Ancient Roman military unit, A band of warriors.  Persons banded together.  Group of persons with a common statistical characteristic. [Latin] .
  • 10.  Similar to the study group in everything except the exposure under the study  Must have a similar chance of the occurrence of the outcome, compared to the study group  Source: general pop., neighbors, friends of cases, hospital…  Matching of factors that might affect the studied relationship (confounding factors)
  • 11.  Baseline information should be obtained from members of the two groups. This information includes: - Exposure factor: to ensure its presence in the study group and its absence in the control group - Outcome: to ensure its absence in both groups - Confounding factors: to assess their presence and level
  • 12.  A rigorous system of follow up should be planned and implemented to avoid losses  The same follow-up maneuver should be done in both groups  In each follow-up, subjects are assessed for the occurrence of the outcome under study (endpoint) which must be: - objectively measurable (hard point) - specific -valid Ascertainment of exposure status in each follow- up visit
  • 13.  The period of follow-up is determined by the nature of the expected outcome and its latency from the exposure, it should be: - not too short: no enough time for the outcome to occur - not too long: probability of the occurrence in the control group approaches that in the study group
  • 14. disease historical follow-up Control group no disease Past (1945) Start FU Time Of The study Today 2007
  • 15. Tim e Of The stud y Start FU Concurrent FU disease No disease Future 2010
  • 16.  Identification and classification of cohort is based on exposure status  Disease is not present at the start of FU  Follow-up is in the direction of the natural history of the disease
  • 17.  Calculation of incidence rates among exposed and non exposed groups  Estimation of risk
  • 18. a+b+ c+d dc ba Diseased Not diseased Yes No a+b c+d b+da+c Total Control (not exposed) Study cohort (exposed)
  • 19.  Incidence rate among exposed= Ie = a/a+b  Incidence among unexposed= Iu = c/c+d  Total incidence= a+c/a+b+c+d
  • 20.  Relative Risk incidence of disease among exposed RR = ______________________________ Incidence of disease among non-exposed a/a+b = _________ c/c+d
  • 21.  Attributable Risk Incidence of disease among exposed – incidence of disease among non exposed AR = _______________________________ Incidence of disease among exposed a/a+b – c/c+d AR = _______________ a/a+b
  • 22. 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
  • 23.  Incidence of lung cancer among smokers 70/7000 = 10 per 1000  Incidence of lung cancer among non-smokers 3/3000 = 1 per thousand 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)
  • 24. Strengths  We can find out incidence rate and risk  More than one disease related to single exposure  can establish cause - effect  good when exposure is rare  minimizes selection and information bias Weaknesses  losses to follow-up  often requires large sample  ineffective for rare diseases  long time to complete  expensive  Ethical issues  Status change with long follow up  Change in diagnosis along FU

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