Though the discussion of the separate layers may give the
impression that they are distinct, their borders are porous. In the real
world there are always constraints that limit what can be tested and
how it can be tested. Sometimes the effects of the constraints can be
anticipated and accounted for in the test design. For example, a
client may specify that the test has to be administered in an hour or
less. The time constraint will limit the number of tasks that can be
administered, which will limit the amount of evidence that can be
gathered, which, in turn, will limit the number of claims that can be
made.