Relative Risk clipped from Wikipedia
http://en.wikipedia.org/wiki/Relative_risk 
Relative risk is used frequently in the statistical analysis of binary outcomes where the outcome of interest has relatively low probability. It is thus often suited to clinical trial data, where it is used to compare the risk of developing a disease, in people not receiving the new medical treatment (or receiving a placebo) versus people who are receiving an established (standard of care) treatment. Alternatively, it is used to compare the risk of developing a side effect in people receiving a drug as compared to the people who are not receiving the treatment (or receiving a placebo). It is particularly attractive because it can be calculated by hand in the simple case, but is also amenable to regression modelling, typically in a Poisson regression framework.
In a simple comparison between an experimental group and a control group:
    * A relative risk of 1 means there is no difference in risk between the two groups.
    * An RR of < 1 means the event is less likely to occur in the experimental group than in the control group.
    * An RR of > 1 means the event is more likely to occur in the experimental group than in the control group.