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Sturgeon's Law is an adage by Theodore Sturgeon. He first used it in 1951.
The correct phrasing is "Ninety percent of anything is crud." It is commonly quoted (or misquoted) when discussing the relative quality of fanworks.
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.'" 
For more information, see Sturgeon's Law at Wikipedia.