<P> A rule of thumb is a principle with broad application that is not intended to be strictly accurate or reliable for every situation . It is an easily learned and easily applied procedure for approximately calculating or recalling some value, or for making some determination . It is based not on theory but on practical experience . Compare this to heuristic, a similar concept used in mathematical discourse, psychology, and computer science, particularly in algorithm design . </P> <P> The exact origin of the phrase is uncertain . The earliest known citation comes from J. Durham's Heaven upon Earth, 1685, ii. 217: "Many profest Christians are like to foolish builders, who build by guess, and by rule of thumb ." </P>

What is the origin of rule of thumb