 Arts ratio is the measure of association in case control study. The first and most important step in calculating arts ratio is deriving the 2 bar 2 table or the contingency table correctly. I have three rules for this. Number one is outcome should be in the top. Exposure will be present in rows. Next rule is positive should be first always then negatives. Third rule is A, B, C, D in this format not A, B, C, D. So if we follow the three rules we will correctly derive the 2 bar 2 table. We can calculate the exposure rates among the cases that is given by A by C and exposure rates among the control is given by B by D. Then we can calculate the arts ratio by A, D by B, C. You can remember by after death divided by before Christ. So now how to interpret this arts ratio? If it is equal to 1, there is no association between exposure and outcome. If the arts ratio is greater than 1, then there is a positive association, which means the presence of the exposure increases the chance of outcome, smoking and alcoholism in case of cardiovascular diseases. If your arts ratio is less than 1, then that is called as negative association or protective or preventive association, vaccination in case of infectious diseases. So arts ratio is a measure of association in case control study given by the formula A, D by B, C.