Static timing analysis is the most used approach to critical path detection in combinational circuits. Since traditional criteria used in standard static timing analysis depend on circuit delay assignment, the analysis has to be repeated after each timing optimization step. A new approach to static timing analysis has been recently proposed in . The cornerstone of the method consists of grouping paths into three categories: strongly perceptible, imperceptible, and weakly perceptible. This categorization allows definite distinction between paths that impact the circuit delay, and therefore need to be sped up, and those that do not, thus reducing the timing optimization effort. Furthermore, this new approach has the advantage of being independent on the circuit delay assignment and, therefore, static timing analysis need not be applied over and over after each timing optimization step. The main drawback of the method is that the criteria to check which category a path belongs to, which has been provided by the author, are partially ambiguous and not easy to compute. We identified new criteria for this path categorization based on the flattening of a circuit into an equivalent normal form (ENF). Our criteria, while being straightforward to check, clear any previous ambiguity, thus allowing the capture of the power of this new path categorization.