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## Glossary of Epidemiology: 14) Control Selection: Primary Study Base

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Fixed accrual and unmatched controls
Common to assume all persons in the study base have the same study window - analgous to fixed/defined followup design
5 - Procedures for using a primary study base
1) Identify every member of the study base. 2) Identify each of the M1 cases. 3) Randomly sample k*M1 controls from non-cases (k is number of controls per case). 4) Ascertain covariates. 5)Ordinary logistic regression
Why Match
It can improve statistical efficiency and control for confounding
Dangers of matching
Over-matchind and unmatched analysis can reduce odds or statistical efficiency
4 steps to category/frequency matching
1) Study base divided into mutually exclusive/exhaustive strata. 2) For each case a stratus of the case is identified. 3) k controls are selected by random sampling (each non-case used once). 4) Repeated for all cases
Individual Matching
each case matched on factors associated with difficult-to-define factors. Use conditional logistic regression
overmatching
factors that are effects of exposure on on the causal pathway should NOT be used for matching
1) Control confounders, more efficient than in analysis. 2) Statistical efficiency if true confounder (5 - 15% variance reduction) 3) Reduce information bias. 4)Feasible way to identify controls
1) cost. 2) exclusion of unmatches cases. 3) Cannot study effect of matching variable. 4) Reduced stat efficiency if exposure variation is reduced.
5) More comples stats analysis
Variable Accrural
used for varaibale time window or accrural period. AKA - Incidence, Incidence density sampling, risk set and risk set sampling
5 Procedures for variable accrural
1) Identify every member of study base. 2) Identify To (begninning) and T1 (end) of study eligibility. For cases, index date ends eligibility. 3) Process the cases in order of the index dates. 4) Exposure, covariates and effect modifiers are measured at index date. 5) Conditional logisitic regression
3 steps to process cases in variable accurual
1) At date Tx - identify all persons who are still eligible (risk set). 2) Randomly select k controls from risk set. 3) Set index date of controls to Tx
Unique componant to variable accrural
A subject who was a control can become a case. The same control can be selected for more than one case.
Number of controls
Depending on cost. Usuall 4 - 5 per case. Do power calculations.