cohort study

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  • Three types of cohort study have been distinguished on the basis of the time of the occurrence of the disease in relation to the time at which the investigation is initiated and continued:
  • cohort study

    1. 1. Cohort study: PROSPECTIVE STUDY, LOGITUDINAL STUDY, INCIDENCE STUDY & LOOKING FORWORD STUDY.
    2. 2. FEATURES OF COHORT STUDY The cohort are identified prior to the appearance of the disease under study The study group so defined are observed over a period of time to determine the frequency of the disease among them. Cause to effect
    3. 3. Concept : by definition, a group of people who share a common characteristic or experience within a defined time period. E.g. age, occupation, exposure to a drug or vaccine, pregnancy. The comparison group may be general population from which the cohort is drown, or it may be another cohort of a persons thought to have had little or no exposure to the substance in question, but otherwice similar
    4. 4. Indications: When there is good evidence of an association between exposure and disease, as derived from clinical observations and supported by descriptive & case control studies. When exposure is rare, but the incidence of the disease is high among the exposure group like those in industries, exposure to x rays ect. When attrition of the population can be minimized, e.g. follow up is easy, cohort is stable, co operative and easily accessible. When ample funds are available.
    5. 5. Frame work Effect to cause…… exposure has occurred but the disease has not. cohort Disease Yes No Total Exposed to Etiological factor a b a+ b Not exposed to Etiological factor c d c+ d
    6. 6. Types of cohort study: Prospective cohort study Retrospective cohort study A combination of retrospective and prospective cohort study
    7. 7. Three types of cohort study have been distinguished on the basis of the time of the occurrence of the disease in relation to the time at which the investigation is initiated and continued.
    8. 8. Prospective cohort study/current The out come not occurred at the time the investigation begins Most study are begin in present and continue into future. E.g. smoking and the lung cancer.
    9. 9. Retrospective study/ historical The out come all occurred before the start of the investigation. the investigation goes back in time, some times 10 yrs to 30 yrs. to select these groups from existing records of past employment, medical records, ect.
    10. 10. A combination of retrospective and prospective cohort study The cohort is identified from the past records, and it is assessed of date for out come. The same record is followed up prospectively in to the future for further assessment.
    11. 11. ELEMENTS OF COHORT STUDY SELECTION OF STUDY SUBJECTS OBTAINNING DATA ON EXPOSURE SELECTION OF COMPARISION GROUPS FOLLOW UP ANALYSIS
    12. 12. SELECTION OF STUDY SUBJECTS GENERAL POPULATION Special groups: A) select groups: B) Exposure group
    13. 13. OBTAINNING DATA ON EXPOSURE Cohort members: personal interview, or mailed questionnaire Review of the records Medical examination or special test Environmental survey
    14. 14. SELECTION OF COMPARISION GROUPS Internal comparison External Comparison with general population rates
    15. 15. FOLLOW UP Regular follow up of participant? Depending on the out come to determined. (morbidity or mortality). Periodic medical examination of each member of the cohort Reviewing physical & hospital records Routine surveillance of death records Mailed questionnaire, telephone calls, periodic home visits……
    16. 16. Drawback: Migration, change of residence or withdrawal from occupation. These loses may bias the result
    17. 17. ANALYSIS The data are analyze in terms of: a) Incidence rates of outcome among and non exposed. b) Estimation of risk: i) relative risk ii) population attributed risk
    18. 18. Advantages Incidence can be calculated Several possible outcomes related to exposure can be studied simultaneously---- i.e. we can study the association of suspected factor with many other diseases in addition to one under study Cohort study provide a direct estimation of relative risk dose response ratio can be calculated Since comparison groups are formed before the disease develops, certain forms of bias can be minimize like misclassification of the individuals in to exposed and unexposed groups.
    19. 19. Disadvantages Large population, they are generally unsuitable for investigating uncommon disease or disease with low incidence in the population. it takes long time to complete the study and to obtain the result ( 20-30 yrs or more in cancer studies) by which time the investigators may died or participants may have change there classification. it is difficult to keep a large number of individuals under medical surveillance indefinitely. Administrative problems such as loss of funding, loss of experienced staff, extensive record keeping are inevitable Lose of interest, migration Selection of comparison groups which are representative of the exposed & unexposed segments of the population is a limiting factor. Expensive Alter people’s behavior
    20. 20. case control cohort study. Effect to cause Start with the disease Test the suspected cause occurs more frequently in those with the disease than among those without the disease Usually the first approach to the testing of a hypothesis, but also useful for exploratory studies Involves fewer number of subjects Yields relatively quick results Suitable for the study of rare diseases Relatively inexpensive Cause to effect Starts with the people exposed to risk factor or suspected cause Test whether the disease occurs more frequently in those exposed, than in those not similarly exposed Reserved for testing of precisely formulated hypothesis Involves large number of subjects Long follow up period often needed, involving delayed results. Inappropriate when the disease or exposure under investigation is rare expensive

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