Design for testability (DFT) works to make a circuit more testable to ensure that it was manufactured correctly. Alfred Crouch explains the purpose of DFT in his book, Design-For-Test for Digital ICs ...
Suppose you observe a sequence of people entering a store and you record the color of shirt worn by each person. You see this sequence of 24 shirts: 0, 0, 3, 3, 2, 1 ...