What is risk?

When scientists talk about risk, they're referring to a probability — the chance that something may occur, but not a guarantee that it will. For example, if you flip a coin, there is one chance in two, or a 50 percent chance, that the coin will land heads up.

Risk estimates for cancer and other diseases are determined by studying large groups of people. Researchers focus on the probability that any person or category of people will develop the disease over a certain period of time. They also look to see what characteristics or behaviors are associated with increased or decreased risk.