scooby doo algorithm

Today I learned, while reading about the new <main> element recently added to HTML5, that the Scooby Doo Algorithm is a thing:

… you always know that the person behind the ghost mask will be the sinister janitor of the disused theme park, simply because he’s the only person in the episode who isn’t Fred, Daphne, Velma, Shaggy, or Scooby.