cecbevwr wrote:I would of course be interested to know what characteristics you believe make a sudoku puzzle easy, medium or hard.?

First, as many has stated, the number of clues isn't directly related to the difficulty of a puzzle. I will agree, though, that in many published (and possibly hand-made) puzzles, the two do appear to be related.

IMHO, the "real" measurement of difficulty is the number of solution paths a puzzle has, and which techniques each step requires. For argument's sake, assume we limit ourselves to the most basic techniques, naked and hidden singles.

If it is, at any given point in the solution process, possible to fill in at least 5 different numbers, the puzzle will probably be rated Easy. However, if there is only one number that is a naked or hidden single, and that opens up for another lonely single, etc., all the way to the end, it's probably going to be rated Hard or Fiendish or something thereabout.

So the number of single-solvable cells for a grid is actually quite unrelated to the number of clues your start with. As just above this post, the harder puzzles often doesn't get hard until about halfway into the solution, which would be illogical if there were a relationship.

One more thing, though... If you take a hard puzzle and add random, not-yet-deductable clues to it, you will also likely add solutions paths to the puzzle (unless you place them carefully in non-critical cells), and it will become easier. Hmm, perhaps that's what the publishers are doing. That would explain the correlation you noticed.

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