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21 Cards in this Set
- Front
- Back
Major problem for controls in secondary study base
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If the Probability of being a control differs for the E+ and E- groups, then the measures of effect will be biased (SELECTION BIAS)
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4 common methods for control selection in a seconday study base
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1)population lists. 2)neighborhood controls. 3)Random-digit dialing. 4)Hospital controls
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Control selection and population lists
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If lost participants from the population list then there is SELECTION BIAS. Eg. (of 1330 eligible controls only 998 participated)
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Neighborhood controls
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Study base is divided into mutually exclusive neighborhoods where all cases are from. Controls are randomly sampled from the neighborhood of each case at the time disease is diagnosed.
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When best to use neighborhood sample
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1) recent or current disease identification. 2) Sufficient exposure variation within neightborhoods.
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Disadvantages of Neighborhood samples
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Lack of exposure variation loses statistical efficiency
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Random Digit Dialing
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Random numbers likely within secondary study base. Strict procedures for qualifying controls. Keep track of tries to get control
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Problems with random dialing
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1) Subjects without phones. 2) Correspondence between telephone numbers and persons. 3) Nonresponse.
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Hospital controls - timing
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Cases are identified continuously in real-time so controls can be selected from patients in the hospital at the same time as cases.
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Identifying hospital controls
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Disease should NOT have the probability of admission related to E. To reduce this some use several diseases
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Biggest mistake for hospital controls
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Using the same organ system to "make controls like the cases"
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When and How to initiating procedures for hospital controls
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Start when a case is identified, have a defined procedure, keep track of tries.
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Hospital controls and index date
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Tx is set at oset of the symptoms for the disease and a comparable time for the controls
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4 advantages for hospital controls
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1) Convenience. 2) High participation rates (especially for acute diseases). 3) Controls in hospital likely to have same source of information bias (have the same data). 4)Same access to medical care.
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2 ways hospital controls are valid
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1) Treatment and referral patters are same for control and case diseases (often in network of hospitals). 2) Control diseases not associated with exposure.
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3 Althernative control selection methods
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1) Medical practice controls. 2)Friend or relative control. 3) Controls outside study base.
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Medical practice controls
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Managed care patients. Can lead to bias (eg. caffeine and colon cancer)
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Advantages of Friend/Relative control
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1) Convenient. 2) Possibly Reduce social class bias
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Disdavantages of Friend/Relative control
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1) Does not meet study base principle. 2) Differential selection related to exposure is likely
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Controls from ouside study base
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Comparable to using historical controls in CT or external E-
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Multiple control groups
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Try to reduce bias. Usually a dark alley (what if results differ between the groups!)
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