In this paper we examine the difficulty of finding an equivalent deterministic automaton when confronted with a non-deterministic one. While for some automata the exponential blow-up in their number of states is unavoidable, we show that in general, any approximation of state complexity with polynomial precision remains $pspace$-hard. The same is true when using the subset construction to determinize the NFA, meaning that it is $pspace$-hard to predict whether subset construction will produce an exponential blow-up'' in the number of states or not. To give an explanation for its behaviour, we propose the notion of subset complexity, which serves as an upper bound on the size of subset construction. Due to it simple and intuitive nature it allows to identify large classes of automata which can have limited non-determinism and completely avoid theblow-up”. Subset complexity also remains invariant under NFA reversal and allows to predict how the introduction or removal of transitions from the NFA will affect its size.