The subject matter most useful in effectively dealing with such uncertainties is contained under the heading of probability. In everyday terminology, probability can be thought of as a numerical measure of the chance or likelihood that a particular event will occur. For example, if we consider the event “rain tomorrow,” we understand that, when the television weather report indicates “a near-zero probability of rain,” there is almost no chance of rain. However, if a 90% probability of rain is reported, we know that it is very likely or almost certain that rain will occur. A 50% probability indicates that rain is just as likely to occur as not.