Absolute Risk Reduction and Excess Risk are commonly referred as Risk Difference. It is generally the difference in the risk between two different activities or treatment.
The risk difference can be between a control group's event rate (CER) and the experimental group's event rate (EER). Commonly calculated between treatments A and B, where A is a drug and B without any drug.
If the experimental group size is 12 and control group size is 13 and the events in experimental group is 2 and control group is 3, then
CVR = c / c + d
CVR = 2 / (2+3)
CVR = 0.4
EER = a / a+c
EER = 12 / 12 + 13
EER = 0.48