The increased risk of dying in a hospital is because the best treatment for most life threatening illnesses is only available in hospitals. Risk is the likelihood of some outcome occurring given the presence of some factor. The outcome is generally referred to as a disease although it might also be death or some adverse medical condition. The factor whose relationship with the outcome we are interested in is generally referred to as the exposure. In the health sciences, the measure of risk we are usually interested in is the Relative Risk, although other measures of risk such as an Odds Ratio might be dictated by the nature of the study.
Research using a case control design is the most common use of an odds ratio.
This calculation requires that information regarding the exposed population is available. This estimation is used when information is lacking for the exposed population, such as in case control studies where the selection of cases and controls is based on a research design which minimizes the impact of other factors on the analysis. A value of one equals unity or an indication of no risk. A value less than one indicates a protective relationship, and a value of greater than one indicates a deleterious relationship. It is important to note that the risk may or may not be statistically significant.