There are two basic 50/50/90 rules. One of them refers to probability, and the other one to time management.

The first one states that in any decision that a human makes which has a 50/50 chance of being correct, that the human will pick wrong 90 percent of the time. This one is often heard of when referring to multiple choice tests, but is also sometimes bandied as a taunt to the loser of a simple coin toss.

The other 50/50/90 rule states that the first half, or fifty percent, of a project will take fifty percent of the time allowed; and the other half of the project will take the remaining ninety percent of the time. This apparent contradiction demonstrates how many projects tend to take far more time than originally intended.