What year did Earl Nightingale say or write this?

Earl Nightingale is credited with saying:





The amount of money you make will always be in direct proportion to the demand for what you do, your ability to do it, and the difficulty of replacing you.|||I've found out that this is recorded in an audio-tape from 1986, titled "Lead the field", which was an extract of his famous radio programs. Anyway,I am not completely sure whether he said this exactly in this year. However, this is the date that appears when you look for this quote on the Internet.


I hope it has been helpful!