exposure and outcome have common causes the exposed are not exchangeable with

Exposure and outcome have common causes the exposed

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exposure and outcome have common causes the exposed are not exchangeable with the unexposed association is not causation [Confounder] Definition of a confounder is: Associated with the exposure (A) in the study base that produce the case Associated with the outcome (Y) in the non-exposed Is not a downstream consequence of exposure or outcome * Not all variables that have these properties are confounders [M-bias] Collider-stratification bias arising from conditioning on a collider that opens a path from exposure to outcome. In the presence of M bias, even if one had stratified on the collider, it may still be possible to estimate a causal effect of exposure on outcome. [Selection bias] Inappropriate selection of controls in a case control study As a result of factors that influence continued participation of subjects in a study. Differential loss to follow-up in longitudinal study Non-response bias, missing data bias Volunteer bias/ self-selection bias Healthy worker bias Conditioning on a common effect procedures used to select subjects factors that influence study participation The relation between exposure and disease is different for those who participate compared to all those who should have been theoretically eligible for the study Any types of study, selection bias can occur
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Case-control: selection of case/controls is dependent on exposure Cohort: success of tracing subject, selection into the study [Collider-stratification bias]
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[Experimental Studies] Investigator assigns exposure Clinical Trial Randomization: Exchangeability (+), Confounding↓ Blinding: ↓Potential for bias Field Trial Primary prevention e.g., vaccine trials Community intervention trial Community intervention studies e.g., fluoride in drinking water [Non-experimental Studies] Observational study: Investigators do not assign exposure Cohort Define 2 or more groups: free of disease, but at risk Measure the occurrence of the disease Start a single group with heterogeneous exposure P-T,IR, CI, OR IRR etc Case- control Identify the cases Controls (sampled independently exposure) Sample from the study base Determine the exposure distribution Estimate relative size of the exposed & unexposed Ratio (OR) Cross- sectional All person in the population At the same time Disease prevalence ×etiologic research preval ence Proportiona l mortality Only dead subjects The Proportion Mortality Ratio (PMR) The proportion of death due to a specific cause among the exposed and among the unexposed PMR Ecological Information on exposure and/or disease on a group level (not individual level) No information on if “exposed” are getting disease Aggregate association may not reflect ind. Assoc. Scatte r plot
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[Prospective vs. Retrospective] Prospective: Exposures are measured before the outcome events occur Retrospective: Exposure are measured after the outcome events occur [Person-Time of Observation] Sum of time at risk in the cohort study for each person Can be computed in closed & open cohorts [Closed & Open Cohorts] Cohor t Closed Open How Membership defining event No New member can enter Lose members: Outcome (death) Membership: defined by “state” Can enter and re-enter Can exit Good CI can be measured directly
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