Risk

Risk is a measure of incidence of a disease. It shows the probability of getting a disease over a certain period of time.

To calculate risk, divide the number of diseased subjects by the overall number of subjects at risk.

a/ a+b

Quantifying Risk

• Odds Ratio (OR)
• Relative Risk (RR)
• Attributable Risk (AR)
• Relative Risk Reduction (RRR)
• Absolute Risk Reduction (ARR)
• Number Needed to Treat (NNT)
• Number Needed to Harm (NNH)

RRR can be used to exaggerate the effectiveness of medications. In patients without heart disease with

high LDLlevels, the use of statin medications may reduce mortality. In generally healthy persons whose

only abnormality is an elevated LDL, there is about a 3 percent mortality rate from cardiovascular disease

over 5 years. With the use of statin medications for 5 years, this is reduced to 2 percent mortality, a

difference of 1 percent; this is an ARR of 1 percent. In other words, you must treat 100 people for 5 years

to save one life. However, the RRR in this example looks much more impressive. Going from 3 percent

mortality to 2 percent mortality is an RRR of 33 percent.