Why is ATPG easy?

Empirical observation shows that practically encountered instances of ATPG are efficiently solvable. However, it has been known for more than two decades that ATPG is an NP-complete problem. This work is one of the first attempts to reconcile these seemingly disparate results. We introduce the concept of circuit cut-width and characterize the complexity of ATPG in terms of this property. We provide theoretical and empirical results to argue that an interestingly large class of practical circuits have cut-width characteristics which ensure a provably efficient solution of ATPG on them