How to clify and create Sudoku games that are easy/medium/hard?

You clify them as to how many different numbers can go in each square at the start, eg easy would be that only 1 number can go in a square over many squares therefore not much guessing and advanced technique is required to solve, medium is that some advanced technique is required but still a few easy squares to fill in. etc etc, i would cl yours as medium definatly.