GUESTIMATE methods are a combination of mathematics, statistics and common sense of getting meaningful results for decision making.
The most famous approach was suggested by Enrico Fermi. Mr. Fermi was convinced that an error of order of magnitude (up to ten times) can be accepted for certain problems.
There are many useful results can be obtained by using probability estimation.
Expected values are largely used in economics, business, finance, and opportunity choice.
Old fashion rule of thumb is a widely used approach among non-mathematicians.
Certain results can be obtained if we consider either limits or average values of variables.