Difficulty rating systems I've seen essentially take a sudoku and then solve it starting with easy human steps, and working their way down to more complicated l

Difficulty rating systems I've seen essentially take a sudoku and then solve it starting with easy human steps, and working their way down to more complicated logical deductions. Chokepoints that can only be found in a large set, or having to solve the puzzle with a complicated deduction like an X-wing or Y-wing, raise the number. With guessing as a fully last resort.