unit 4 pt 2 - Obtaining Exposure Information Once you have...

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Case Control Studies Part 2 Dr. H. Stockwell NO AUDIO ON THIS SLIDE. CLICK NEXT TO CONTINUE Obtaining Exposure Information ± Once you have identified cases and controls, the next step is to obtain exposure information from both groups ± Exposures occurred in the past – often many years ago many years ago Collecting information on past exposures ± Often collect information by interview ± Interviewee may not know information or may not remember it well. ± Poor memory of past exposures can lead to misclassification Sources of Exposure Information Self-report: Person-to-person interview; telephone interview; self administered questionnaire or a combination Medical records • (HMO; County records, MD offices, hospital records) Records completed before the occurrence of Records completed before the occurrence of outcome event preferable) Interviews with proxies/surrogate Interviews with proxies/surrogates Spouses, sibs, friends Collecting Exposure Information ± Will cases and controls remember with equal accuracy? ± Will they report accurately? ± Will proxy respondents answer as the intended respondent would have answered? Recall Bias ± Differing recall by cases than controls is named recall bias ± Cases may be more motivated to try to remember things in their past that might have caused their disease than are (healthy) controls ± Few examples in literature showing Few examples in literature showing major impact on study findings
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Observer Bias ± As interviewer may know who is a case and who is not, they may treat the cases and controls differently when collecting exposure information ± Can lead to interviewer/observer bias Advantages ± Good for studying rare diseases ± Relatively fast to conduct (months/years) ± Relatively inexpensive Relatively few subject ± Relatively few subjects ± Existing records may be available ± Minimal risks to subjects usually ± Study multiple causes of disease (exposures) ± Evaluation of diseases with long latency periods Disadvantages ± Selection of appropriate controls may be difficult ± Relies on recall or existing records ± Validation of information difficult ± Control of other variables may be difficult ± Cannot calculate relative risk directly or determine prevalence ± Temporal sequence between E and D difficult to establish ± Prone to Selection/Observer/Recall Bias Measure of Association ± Need a measure of the association between disease and exposure ± In case-control studies the measure is called the Odds Ratio ± May be used on its own or as an estimate the Relative Risk Calculating Risk in Case-Control Studies ± Cohort studies calculate relative risk To calculate relative risk must have ± To calculate relative risk must have incidence ± In a case-control study do not have incidence – just selected cases and controls ± Case-Control study cannot calculate relative risk directly-use ODDS RATIO to estimate Odds Ratio ± Odds is the number of ways an event can
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This note was uploaded on 07/15/2011 for the course PHC 6000 taught by Professor Staff during the Summer '08 term at University of South Florida - Tampa.

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unit 4 pt 2 - Obtaining Exposure Information Once you have...

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