The only part of the process that I'm not confident about is the control.

Say I compare Slinker and Stinker, and they have almost exactly the same average sentence length, FOG readability index and so on, how do I know I wouldn't get the same result comparing Slinker with Ernest Hemingway or Toni Morrison or Irvine Welch?

You'd need to be able to say with confidence that if author A scores a 97% similarity score with author B, then you couldn't get the same result with author X.

The mis-spellings ought to be quite easy to implement though:

Author A makes the following mistakes every time.
Author B makes the following mistakes every time.

That would convince me...
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“Every bit of code is either naturally related to the problem at hand, or else it's an accidental side effect of the fact that you happened to solve the problem using a digital computer.” M-J D