Controllability and observability
Beginner
How easy it is to set a spot inside the chip to a value, and how easy it is to see what value it holds.
Novice
Controllability is how easily a test can force a point inside the circuit to 0 or 1 from the chip’s pins. Observability is how easily the value at that point can be seen at an output. Scan makes every flip-flop fully controllable and observable.
Expert
Quantified by testability measures such as SCOAP, which ATPG uses to steer its search and which guide test-point insertion. Deep sequential state and logic that random patterns rarely exercise score poorly on both.