Losses caused by occurrences of unexpected events are problems both for individuals and for society as a whole. Insurance is a mechanism for spreading these losses. Examples of insured events and their con- sequences are property damage due to fire, theft, flood, hail, or car accident (replacement cost); disability or death (loss of future income and support); illness (cost of medical treatment); and personal injury resulting from accidents or medical malpractice (cost of treatment and personal suffering). These are just a few of the many insured events, and we will assume that most readers understand the basic ones, which are used as illustrations throughout this book. The more complicated insurances will be described at the time of their use.
Clearly, the actuary wants to know something about the probability of an insured event occurring. In particular, it is important to know the expected number of occurrences for a specific measure of exposure to the risk. For a simple example, we might observe the number of claims occurring during the next year for a certain group of insured cars. Upon dividing by the number of cars, we obtain an estimate of the expected number of claims for one car in one policy year. This ratio is the mean frequency and is defined as