After shaking it, and making a number of other attempts at finding a way to use it, she gave up and asked me how it worked. She also did not intuit that the large raised area on top was a button. But once I pointed out that the cursor moved when the mouse was moved on the desk's surface and that the raised area on top was a pressable button, she could immediately use the mouse without another word. Given this insight, the measure of intuitiveness used by the magazine usability labs is logical. The percentage of tasks completed increases with increasing intuitiveness since it simply means that the user is already at least partially trained with respect to the feature or set of features under test.
Training tends to decrease the time required for task completion, especially at first.
Similarly, the number of times the user has to reference help screens increases with decreasing intuitiveness, that is, decreasing familiarity. Even where my proposals are seen as significant improvements, they are often rejected nonetheless on the grounds that they are not intuitive. The client wants something that is significantly superior to the competition. Therefore it cannot be intuitive, that is, familiar. What the client usually wants is an interface with at most marginal differences that, somehow, makes a major improvement.