"Obviously, when probability is subjectively determined and not based on the past data, the different individuals will attach different probabilities to the occurrence of various outcomes and therefore they will make different choices."
-
Risk refers to the situation when there is more than one possible outcome of a decision and the probability of each outcome is either known or can be estimated.
Therefore, to measure the degree of risk we need to know the probability of each possible outcome of a decision.
The probability means the likelihood of occurring of an event.
http://www.economicsdiscussion.net/articles/measuring-risk-probability-of-an-outcome-explained-with-diagram/1368