Occurs when referral patterns within a community causes the hospital population to be a misrepresentation (either overrepresentation or underrepresentation) of the general population.

Referral Bias

Incomplete ascertainment of risk factor in control group leads to underrepresentation of the factor, and an inaccurate study.

Ascertainment Bias

Also called admission-rate bias, refers to the fact that people with more than one conditions are more likely to be admitted than people with just one condition. This increases the likelihood of the chosen risk factor to be present - which causes an overrepresentation in the hospital sample.

Berkson Bias

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

The paper focusses on case control studies done in a clinical setting. Thus it expands on types of biases generated in this environment. 
The types elucidated are : 
1. Referral Bias
2. Ascertainment Bias
3. Berkson Bias
4. Hospital Control Bias

University of Michigan - Ann Arbor

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 .) 

Assessing bias in case-control studies.

Determining the relationship between risk factors and diseases (trait).

Motivation of the study

Illustrate how improperly selected cases/controls can generate biases in the study. Explore types of Bias generated during clinical studies.

Purpose of Paper

Types of Bias outlined in paper

The odds ratio is used to measure the association between a factor and a trait in a case controlled study. 
1. An odds ratio >1 implies that the presence of the factor increases the probability of the existence of the trait.
2. An odds ratio <1 implies the opposite i.e. presence of factor inhibits presence  of trait. 
3. An odds ratio =1 implies no impact of the factor on the trait. 

Odds Ratio

Several key conclusions emerge regarding the assessment of bias in case-control studies. First, bias can mask true associations and form false ones, potentially invalidating research findings. Second, there is no universal method to identify all biases; rather, the process is intrinsic to the specific research problem and must be addressed during the study's design phase. Finally, case-control studies are especially susceptible to biases because errors can occur in the selection of either the cases or the controls, both of which can adversely impact or entirely invalidate a study.

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