The decision maker does not know, or cannot estimate, the probability of occurrence of the possible outcomes.
It is more difficult than making decision under certainty because there is insufficient information.
A decision made under risk (also known as a probabilistic, or stochastic) is one in which the decision maker must consider several possible outcomes for each alternative, each with a given probability of occurrence.
Risk analysis is a decision-making method that analyzes the risk (based on assumed known probabilities) associated with different alternatives.