While reviewing text in an online course, I was confused by the usage of the the phrase "is preferable than". I could not find a reference to the proper use of "preferable than" and "preferable to". Could you please identify a reference source that could resolve this issue?
The course is discussing Java Servlet applications and the direct text is "Using a controller is preferable than embedding code in multiple views because that may lead to an error-prone environment."
There are a host of examples for the "be + preferable + to" but I know that "be + preferable + than" is appropriate in certain contexts. I was wondering if there was a 'rule of thumb' that might make it easier to relate to others.
Is it acceptable to use 'half' of an irregular compar[a]tive.
'is preferable than' is ungrammatical.
Possible reasons for the error: omitting "more"
 It's a typographical error. The writer needs a proof-reader.
 It's a language error. The speaker inserts any suffixed-formed adjective here: be + ______ + than; e.g., given the structure "is bigger than", therefore, by analogy, *"is preferable than."