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Cohort and case con revised

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Cohort and case con revised

  1. 1. By Dr Aijaz Ahmed Sohag Prep by: Abdul Wasay Baloch Amna Inayat Medical College Case Control vs Cohort Control
  2. 2. Case Control Cohort Control  Proceeds from effect to cause  Starts with the disease  To know suspected cause occurs more frquently having disease than those without disease  Suitable for study of Rare disease  Relative inexpensive  Proceeds from cause to effect  Starts with exposure  To know whether disease occur more frequently in those exposed to risk factors(a+b) than non exposed (c+d)  Suitable for exposure of Rare response  Comparatively expensive Case Control Cohort Control
  3. 3.  Generally yields only estimate risks (odds ratio)  Time of study relatively short  Population size needed relatively small  Potential bias larger (assessment of exposure)  Generally yeilds relative risk, attributable Risk, besides incidence rate  Relatively large  Comparatively mimic less (assessment of outcomes)  E.g.  Smoking and lung cancer. Fermingham heart study, Oral contraceptive and health Case Control Cohort Control
  4. 4.  RR estimate may increase or decrease as a result of bias a) Bias due to confounding (may be reduced by Matching) b) Memory or Recall bias c) Selection bias d) Berkesonian bias or Joseph bias e) Interviewer bias (may be reduced by double blinding)  Examples  Adenocarcinoma of Vagina  Oral contraceptive and thromboemolic disease  Thalidomide tragedy  Ethical problems minimal  Incidence can not be measured, and can only estimate the Relative Risk
  5. 5. Incidence Rate Cigrette smoking Developed Lung cancer Did Not Develop Lung cancer Total Yes 70 (a) 6930 (b) 7000 (a+b) No 3 (C ) 2997(D) 3000 (c+d)
  6. 6.  Incidence Rate among Smokers a/(a+b) = 70/7000*1000 = 10 per 1000  Among Non- smokers = c/(c+d)= 3/3000*1000 = 1 per 1000  Estimation of Risk:  Relative risk = incidence of dis among exposed/incidence of dis among non expose= RR = 10/1=10  Attributable risk= incidence of dis among exposed – incidence of disease among non exposed/incidence rate among exposed =10-1/10 *100 = 90%
  7. 7. RR AR  To study etiology (cause) it has an edge over AR  It measure Strength of association between suspected cause and effect  Larger the RR, stronger the association between Cause and Effect. RR 1 indicates no association  RR has less public health importance as does RR  E.g. if RR is 10, it means smokers are 10 times at greater risk of developing lung cancer than non smokers.  Comparatively AR has less importance studying antilogy of disease  It measures To What extent disease under study Attributed to exposure e.g. 90 % lung cancer in smokers was due to smoking  AR gives better idea than does RR of the impact of preventive/public health program in reducing problem

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