Select sample with wide range of admission reasons. This negates any effects that could potentially surface in targeted samples.

University of Michigan - Ann Arbor

If the cause of admission to the hospital and the risk factor are related, this could skew the study and cause an overrepresentation of exposed factors. 

Hospital Control Bias

Sutton-Tyrrell, K. (1991). Assessing bias in case-control studies. *AHA Journals*. 22(7),  938-939 from [www.ahajournals.org/doi/pdf/10.1161/01.STR.22.7.938 .](https://www.ahajournals.org/doi/pdf/10.1161/01.STR.22.7.938 .) 

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