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When an event occurs more or less frequently, we often talk about the frequency of rather than the probability of . Rather than asking “What is the probability of event ,” we may ask, for example, “How frequently does event occur?”

Fatal traffic accidents occur many times per year, and we may record the number of such accidents during a period of length . A fatal traffic accident is understood as an accident where one or more persons are killed. The frequency of fatal traffic accidents in the time interval is given by

(2.6)

The “time” may be given as calendar time, accumulated operational time (e.g. the accumulated number of hours that cars are on the road), accumulated number of kilometers driven, and so on.

In some cases, we may assume that the situation is kept unchanged and that the frequency approaches a constant limit when . We call this limit the rate of the event and denote it by :

(2.7)

In the frequentist interpretation of probability, parameters like have a true, albeit unknown value. The parameters are estimated based on observed values, and confidence intervals are used to quantify the variability in the parameter estimators.

Models and formulas for the analysis may be found in Appendix A.