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Bias and confounding in Cohort and case control study


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Bias and confounding in Cohort and case control study

  1. 1. Presented by :Ikram Ullah BS MLT,2nd batch KMU, Peshawar. 5/17/2017 1
  2. 2. Contents  Bias and its types  Confounding  Bias in cohort study  Bias in case control study  Elimination of bias  Control of confounding  References 5/17/2017 2
  3. 3. Quiz The analytical study where population is the unit of study? a) Cross sectional b) Ecological c) Case-control d) Cohort  A longitudinal or prospective study is also referred to as a) Ecological study b) Cross sectional study c) Cohort study d) Observational study5/17/2017 3
  4. 4. Bias  “Any systemic error (design, data collection, analysis or reporting of a study) in epidemiological study that results in incorrect the estimation of the association between exposure and outcome”  “Deviation of results or inferences from the truth” 5/17/2017 4
  5. 5. Types of bias  1. Selection bias • Occurs when the two groups being compared differ systematically • That is, there are differences in the characteristics between those who are selected for a study and those who are not selected 5/17/2017 5
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  7. 7. Types of selection bias i. Volunteer bias ii. Berskson’s bias iii. Exclusion bias iv. Inclusion bias v. loss/withdrawal to follow up vi. Non response bias vii. Healthy worker effect viii.Selective survival 5/17/2017 7
  8. 8. 2.Information bias  Method of gathering information is inappropriate and yields systemic errors in the measurement of exposure or outcome 5/17/2017 8
  9. 9. Types of information bias i. Observer bias ii. Recall bias iii. Horthorne effect iv. Surveillance bias v. Lead time bias vi. Misclassification bias a) Differential b) Non differential 5/17/2017 9
  10. 10. Confounding  A situation in which the measure of effect of exposure on disease is distorted because of the association of the study factor with other factors that influence the outcome  Confounder must be……. 1. Risk factor for the disease independently 2. Associated with exposure under study 3. It should not be in the direct chain or link between the exposure and outcome5/17/2017 10
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  12. 12. Bias in cohort study 1. Selection bias : Select participants into exposed and not exposed groups based on some characteristics that may affect the outcome.  Loss to follow up bias: Some subjects in any case are likely to be lost to follow up/drop out e.g. smoking and lung cancer, loss to follow up bias occurs if smokers who have lung cancer are more likely to be lost(die) than non smoker with lung cancer 5/17/2017 12
  13. 13. Cont…. • Cross over bias: This may happen because those having the exposure (e.g. smokers) may cross over to the non exposed group(i.e. become non smokers)and vice versa. • Healthy worker effect: Healthy people remain workers, whereas those who remain unemployed, retired or disabled are a group of less healthy 5/17/2017 13
  14. 14. Cont…. 2. Information bias: • Observer bias : This occurs because the investigator is aware about the fact as to which subject is ‘exposed’ and who is not exposed. For obviating this, if possible, ‘blind’ the observer to the exposure status, the details of exposure being known only to another co - worker 5/17/2017 14
  15. 15. Cont….  Measurement bias: Collect different quality and extent information from exposed and not exposed groups  For eliminating this , inform all the subjects of both groups well in advance of the dates and timings of medical examination  Ensure that both the groups are examined by observer who have similar type of training and using similar type of technique and instrument5/17/2017 15
  16. 16. Bias in case control study 1. Selection bias:  Selective survival: Only surviving subjects availabe to be studied; those surviving differ from those dying potentially important ways.  Solution: rapid case ascertainment and interview 5/17/2017 16
  17. 17. Cont…  Selection of inappropriate cases or control: Cases or control who do not have adequate chances of exposure .  For example in a study of OC use (exposure) and thrombophlebitis (cases),cases or control who have undergone hysterectomy or using some other contraceptive , will not have adequate chances of exposure 5/17/2017 17
  18. 18. Cont….  Information Bias:  Occurs due to flawed data collection procedures.  Types of Information bias –  Recall bias  Interviewer bias 5/17/2017 18
  19. 19. Cont….. Recall bias : Cases who are aware of their disease status may be more likely to recall exposures than controls e.g. mothers who have given birth to babies with congenital malformation, mothers with normal babies 5/17/2017 19
  20. 20. Cont…. • Interviewer bias: When interviewer is not blinded knows case status of subjects there is potential for interviewer bias. 5/17/2017 20
  21. 21. Cont…..  Confounding bias: 5/17/2017 21
  22. 22. Eliminating of bias  Representative sample  Well defined structure research  Blinding  Randomization 5/17/2017 22
  23. 23. Control of confounding  At design stage: 1. Restriction: Subject chosen for study possessing a narrow range of characteristics . e.g. restrict study to women having a least one child. 2. Matching: For each patient in one group there is one or more patients in comparison group with same characteristics, except for the factor of interest. E.g. age ,sex ,race etc. 5/17/2017 23
  24. 24. Cont… 3. Randomization: Subjects of study are randomly selected to even out unknown confounders. At analysis stage: 1. Stratification: The process of separating a sample into several sub-samples according to specified criteria such as age ,sex etc. 2. Multivariate analysis: The statistical analysis of data collected on more than one variable. E.g. people age ,weight, body fats .5/17/2017 24
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