1 / 23

COHORT STUDY

COHORT STUDY. Dr Santosh K Yatnatti. EPIDEMIOLOGY: Study Designs Ecological Studies Cross sectional (Prevalence) studies Case-Control Studies Cohort Studies (Prospective, Longitudinal, Incidence) Intervention Studies (experiments). Hypothesis.

greenlee
Download Presentation

COHORT STUDY

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. COHORT STUDY Dr Santosh K Yatnatti

  2. EPIDEMIOLOGY: Study Designs • Ecological Studies • Cross sectional (Prevalence) studies • Case-Control Studies • Cohort Studies (Prospective, Longitudinal, Incidence) • Intervention Studies (experiments)

  3. Hypothesis Medical school performance has no effect on the future career of a doctor

  4. Introduction • What is a cohort • Analytical study • Starts with people free of disease • Assesses exposure at “baseline” • Cohort is followed up • Proceeds from cause to effect • Assesses disease status at “follow-up” • two measurement (baseline and follow-up) • individual is the unit of observation and analysis

  5. DESIGN OF A COHORT STUDY TIME direction of enquiry disease exposed Population People without the disease no disease Some sort of sample disease Not exposed no disease

  6. Cohort study Framework Disease Y N a b a+b Y Exposed cohort Exposure c d c+d N Non-exposed cohort a+c b+d N

  7. Type of Cohort studies • Prospective: start with exposed and unexposed groups, wait for the outcome

  8. Type of Cohort studies • Retrospective: both exposure and outcome have already happened, but groups are still made based on exposed or unexposed Past Present Disease + Disease - Risk + Disease + Disease - Risk -

  9. Criteria of a cohort • Free of disease even subclinical • Both groups equally susceptible • Both groups comparable • Diagnostic and eligibility criteria defined beforehand

  10. Cohort studies • Studies whether exposure to a “risk factor” is associated with a subsequent “outcome” • Select two populations who seem the same except for the hypothesized risk factor • Follow them and see how many have the outcome or disease • Important concept: Start with the risk, then look for the outcome

  11. Elements of cohort study A. Selection of study subjects • The general population (or a sample of it) • Framingham heart study • A group that is easy to follow-up • Doll and Hill study • With some special exposure (especially rare exposures) • Occupational exposures

  12. Elements of cohort study B. Obtaining data on exposure • Interviews or mailed questionnaires • Records • Medical examination • Environmental surveys • Data collected should classify the cohort on presence or absence and degree of exposure

  13. Elements of cohort study C. Selection of comparison groups • Internal comparison • Stratify by level of exposure • Separate cohort • As similar as possible to exposed cohort • education, income, gender, race, geographic location • Comparison with available population rates

  14. Elements of cohort study D. Follow up • Periodic medical examination • Reviewing records • Death records • Home visits, questionnaires, telephone calls

  15. Elements of cohort study E. Analysis Disease Y N a b a+b Y Exposed cohort Exposure c d c+d N Non-exposed cohort a+c b+d N • Incidence among exposed • Incidence among non-exposed • Relative risk • Attributable rsk

  16. FRAME OF COHORT STUDY

  17. INCIDENCE OF DIESEASE (DEATH ) AMONG EXPOSDED RELATIVE RISK = INCIDENCE OF DISEASE (DEATH) AMONG NON - EXPOSED RR OF LUNG CANCER = 10 / 1 = 10 SMOKERS ARE 10 TIMES AT GREATER RISK OF DEVELOPING LUNG CANCER THAN NON SMOKERS

  18. ATTRIBUTABLE RISK: • EXPRESSED AS PERCENT INCIDENCE OF DISEASE RATE AMONG EXPOSED MINUS- INCIDENCE OF DISEASE RATE AMONG NON - EXPOSED x 100 INCIDENCE RATE AMONG EXPOSED = 10- 1 x 100 = 90% 10 TO WHAT EXTENT THE DISEASE UNDER STUDY CAN BE ATTRIBUTED TO EXPOSURE

  19. determination of incidence and risk multiple exposures and multiple outcomes can establish cause - effect good when exposure is rare temporal relationship clear minimizes selection and information bias validity affected by losses to follow-up often requires large sample ineffective for rare diseases long time to complete expensive subject attrition confounding by other factors possible Cohort studies Strength Weakness

  20. Potential biases in cohort studies • Selection (confounding) bias: have to match similar groups • Detection bias: measurement of outcomes needs to be objective and similar for both groups • Length-time bias: study has to be long enough for outcome to happen • Excursion bias: subjects may disappear or drop-out (lost to follow-up)

  21. To Summarize Define the Cohort Selection of study population Get baseline exposure measurements Follow cohort for disease / outcome Additional exposures measured over time Analyse disease risk according to exposure

  22. DIFFERENCES B/W CASE CONTROL & COHORT STUDIES

  23. Thank you

More Related