Puzzle Difficulty Puzzle DifficultyThere are a number of factors that determine puzzle difficulty:
Traditionally Sudoku puzzles are assigned a difficulty of some number of stars up to a maximum of 3, 4, or 5. Comparing stars from different sources shows very little consistency. Even from the same source, the number of stars are only a very rough indicator of difficulty, and often a user will find a 4-star puzzle easier than a 2-star puzzle.
Each puzzle solver has a unique ability to easily recognize certain types of patterns, and will have their own favorite strategies. Assigning a single numerical measure of difficulty to a puzzle is therefore somewhat arbitrary and cannot be accurate for all people. We have attempted to solve this problem in several ways.
First, the Zudoku difficulty number represents the total number of "glances" (visually inspect a cell for a certain pattern or condition) required to scan all unsolved cells for all possible numbers for all possible strategies to find a solution. It is assumed that numbers are tested from 1 to 9; that groups are scanned by row, then column, then box; strategies are tried from easiest to hardest, starting over from the easiest after every change is made; and that the user is adept at all strategies.
Second, a record is kept of the number of times each strategy must be used to solve a puzzle. For the user who has a preference for certain strategies, this may be important along with the difficulty number.
Zudoku can calculate the difficulty of any puzzle using Menu/Tools/Measure Difficulty. This has already been done for the built-in puzzles. The result is normalized to the range of 1 to 100, with 100 itself being reserved for puzzles that require guessing to find a solution.