We have looked at two interfaces, one of which will take about 5 seconds to operate and the other of which will take more than 15 seconds. [...] It is clear which of the two better satisfies the requirement. The next question that we ask is how fast an interface [...] can be. [...] To make a reasonable estimate of the time that the fastest possible interface for a task would take, we can proceed by first determining a lower bound on the amount of information a user has to provide [...] this minimal amount is independent of the design [...]
The information-theoretic efficiency E is defined as the minimum amount of information necessary to do a task, divided by the amount of information that has to be supplied by the user. [...]
E can be zero when the user is required to provide information that is totally unnecessary.
At this point, Raskin shows an example from (what else?) Microsoft Word, which needs no information to proceed, but requires the user to provide some. 0/x = 0.
Do not follow Microsoft's example.
The book is good, particularly chapters 2-4, which are about information efficiency, time efficiency, likelihood of user errors, and similar topics.