"According which criterion is used" grammatically correct?

[tainturier]

Hi,
I handed in a doctoral essay with the title "understanding recovery rates.... according which criterion is used: threshold or clinical recovery" having been told I should not include a "to" between the "according" and "which", yet I've received it back with big red circles from the marker around it saying it's grammatically incorrect.
Is it? I've seen it used in this way in published papers and would like an explanation as to why it either is or is not correct?
Many thanks!

[Alan]

Incorrect.

'According to' is the phrasal prepositional you require.