Pareto principle http://en.wikipedia.org/wiki/Pareto_principle (also known as the 80–20 rule, the law of the vital few, and the principle of factor sparsity) states that, for many events, roughly 80% of the effects come from 20% of the causes
Parkinson's Law http://en.wikipedia.org/wiki/Parkinson%27s_law Parkinson's law is the adage that "work expands so as to fill the time available for its completion".