| http://www.w3.org/ns/prov#value | - Seventeen of the case-control studies were population based (either nested in a cohort or using methods like random digit dialing to ensure that controls are a sample of the underlying cohort that gave rise to the cases)[21,22,24,27,28,30,36,39,44,54,59-61,63-66] whereas the other seventeen were hospital based (using non-cancer hospital controls) [23,26,32-35,37,38,42,45,47-49,51,53,55,56,62].
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