We want to find the ratio of 'cases-with factor to cases-without factor' and 'control-without factor to control-with factor'. In the table this is determined as 
$$\frac{A}{B}$$
and 
$$\frac{C}{D}$$
We then calculate the odds ratio using 
$$\frac{AD}{BC}$$

Explanation of odds ratio table

University of Michigan - Ann Arbor

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

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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