Logical effort
Beginner
A score for how much harder a gate has to work than a plain inverter to do the same job.
Novice
The ratio of a gate’s input capacitance to that of an inverter that can deliver the same output current. An inverter scores 1; a NAND2 scores 4/3; a NOR2 scores 5/3.
Expert
in the delay model (Sutherland, Sproull and Harris). It captures the cost of the series stacks a function requires. Path effort and the best stage count follow from it, which makes it a fast way to compare gate topologies.