Frequency of transition is the expected number of occurrences of a particular transition per unit time (at a specified time or at a steady state). This may be useful to know the total number of occurrences of an event (transition) within a specified time. Frequency of a transition from state to state can be found by multiplying PI(t) and tij (transition rate). For example, consider the system shown in Figure 8-6. The frequency of transition from state 2 to state 1 is P2(t)μ. The expected number transitions can be found by integrating the frequency of transition over a specified interval.