“Ninety percent of most magic merely consists of knowing one extra fact.”

Spoken statement (c. 1903); published in Harper's Monthly (September 1932).
Variants:
None of my inventions came by accident. I see a worthwhile need to be met and I make trial after trial until it comes. What it boils down to is one per cent inspiration and ninety-nine percent perspiration.
Statement in a press conference (1929), as quoted in Uncommon Friends: Life with Thomas Edison, Henry Ford, Harvey Firestone, Alexis Carrel & Charles Lindbergh (1987) by James D. Newton, p. 24.
Variant forms without early citation: "Genius is one percent inspiration and ninety-nine percent perspiration. Accordingly, a 'genius' is often merely a talented person who has done all of his or her homework."
"Genius: one percent inspiration and 99 percent perspiration."
1900s
Variant: Genius is one per cent inspiration and ninety nine perspiration.

Prayers For The Assassin (2006)

Source: Invincible

The Dead Zone (1979)

What Time Is It? You Mean Now?: Advice for Life from the Zennest Master of Them All, Simon and Schuster, 2003, ISBN 0743244532, p. 45. This line has been attributed to Berra and also to Philadelphia Philles manager Danny Ozark. However, it was actually first said by Major League reserve outfielder Jim Wohlford, to whom the line was attributed in April 1974. See Devin Rose, Words of Wisdom - Former Big Leaguer Jim Wohlford - Took the words right out of his mouth http://articles.chicagotribune.com/2003-09-21/features/0309210321_1_notable-quotables-words-contracts, Chicago Tribune (September 21, 2003) (Retrieved March 4, 2016.) and Website of etymologist Barry Popik, Entry dated September 23, 2015 http://www.barrypopik.com/index.php/new_york_city/entry/baseball_is_ninety_percent_mental_and_the_other_half_is_physical/. (Retrieved March 4, 2016.)
Disputed
Variant: Ninety percent of this game is mental, and the other half is physical.
Source: The Yogi Book : I Really Didn't Say Everything I Said

Source: Attributed from posthumous publications, Dialogues of Alfred North Whitehead (1954), Ch. 29, June 10, 1943.
Context: Ninety percent of our lives is governed by emotion. Our brains merely register and act upon what is telegraphed to them by our bodily experience. Intellect is to emotion as our clothes are to our bodies; we could not very well have civilized life without clothes, but we would be in a poor way if we had only clothes without bodies.

Source: The Secret

Source: Synners (1991), Chapter 32 (p. 389)