|See also:||Attitudes Toward Fanfiction|
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The correct phrasing is "Ninety percent of anything is crud." The phrase is often misquoted as "90% of anything is crap," and "90% of things are crap and 10% are brilliant," and a variety of other variations.
One of those variations: "Sure, 90% of science fiction is crud. That's because 90% of everything is crud." Scholars disagree on what word Sturgeon originally used. In its first appearance in print it was "crud," but someone present remembers that in the speech it was 'crap'."  Another fan remembers it as "shit." 
The first known use of "Sturgeon's Law" was in 1963. The "law," however, was cited in Venture Science Fiction in 1957 as "Sturgeon's Revelation." 
For more information, see Sturgeon's Law at Wikipedia.