
Software Reliability ◾ 505
PL =
=
∏
ft ft ft ft
i
i
i
() () ()
3
3
4
4
5
5
5
Similarly, we can compute the sequence of prequential likelihood val-
ues of other prediction systems. For example, an exercise at the end of this
chapter asks you to calculate PL for the prediction system where the estimate
of the mean time to the next failure is the average of the three previously
observed values.
To see how prequential likelihood allows us to compare models, sup-
pose we have two candidate sets of predictions, from prediction systems A
and B. For each prediction system, we compute the respective prequential
likelihood functions, denoting them as ...