The Wild-Ass-Guess (WAG) is used for an activity that has no precedent. It draws on simple rules-of-thumb instead of breaking down specific facets of a project to build a more realistic estimate. If scientists want to build a ship that will take people to Mars and back, they can work on calculating the variables and foresee potential hiccups but in the end, because there is no precedent to draw from, a large part of the estimating becomes a wild guess entirely reliant on rules-of-thumb instead of smart estimates.
