Source : Free On-Line Dictionary of Computing

Ninety-Ninety Rule
     
         "The first 90% of the code accounts for the first 90%
        of the development time.  The remaining 10% of the code
        accounts for the other 90% of the development time".
     
        An aphorism attributed to Tom Cargill of Bell Labs, and
        popularised by Jon Bentley's September 1985 "Bumper-Sticker
        Computer Science" column in "Communications of the ACM".  It
        was there called the "Rule of Credibility", a name which seems
        not to have stuck.
     
        [{Jargon File}]
     
        (1995-07-14)