126 Model-Based Testing for Embedded Systems
among which passage should be quick and easy (or virtually impossible). Trajectory entropy
provides a measure of the uncertainty in selecting a path from a set of paths. A variation on
the techniques of Ekroot and Cover (1993) produces the measure of specification complexity
(Walton and Poore 2000). Trajectory entropy is the sum of the uncertainty of the first step
in the path plus the conditional uncertainty of the rest of the path, given the first step.
This value is the ratio of the source entropy to the stationary probability of the initial
state and is used as an index of specification complexity, the minimum average number of
yes–no questions one would have to ask to identify the path taken. When 2