Grading Sudoku Puzzles is very subjective. It differs from puzzle maker to puzzle maker. My research has revealed what Sudoku patterns you are likely to find in different grades of puzzles and games.
As far as I know, there aren’t any universal standard Sudoku grading rules that Sudoku puzzle and game makers subscribe to.
Simple Sudoku, a game created by Angus Johnson allows for more complex Sudoku patterns than you will find in Pappocom Sudoku by Wayne B Gould. Sudoku puzzles you find in some UK newspapers are supplied by Puzzles By Pappocom. Therefore these puzzles tend to be simpler to solve than some you will find on Sudoku Essentials.
For this article, I used Simple Sudoku to generate the puzzles. I then used the hint function to have Simple Sudoku solve the puzzle it generated.
I did this for several puzzles for each of the five Sudoku grades (Easy, Standard, Hard, Expert, and Extreme). Tabulating the patterns for each grade, gives us an idea how grading Sudoku puzzles is done, at least in Simple Sudoku.
Grading Easy Sudoku Puzzles
As you might expect, an Easy Puzzle Grade has the simplest patterns. I was able to only discover two patterns. They are naked singles, and hidden singles.
If you are unfamiliar with these terms, see my article Sudoku Tips where I discuss most of the patterns I will mention in this article.
A puzzle that only has naked and hidden singles can be solved easily using the scanning method and penciling in all cell values.
Every naked single cell will be revealed when you pencil in or mark up your puzzle. The scanning method will solve for hidden singles.
Grading Standard Sudoku Puzzles
A standard grade, or Medium as I classify them, revealed five patterns. They are naked singles, hidden singles, naked pairs, and two variations of locked candidates.
I suspect, had I generated more puzzles, hidden pairs would also fall into this grade. It is more rare to have a puzzle that has hidden pairs. Hidden singles occur much more often.
Grading Hard Sudoku Puzzles
A hard Sudoku puzzle, I have found, contains these patterns – naked singles, hidden singles, naked pairs, naked triples, naked quads, and two variations of locked candidates.
Again, I expect you might also find hidden pairs, hidden triples, and hidden quads had I run a larger puzzle sample.
Hard sudoku puzzles is where I find the challenge starts.
Grading Expert Sudoku Puzzles
An expert grade, or very hard if you prefer, adds more difficult patterns. Not only, do you have every pattern mentioned above, but now you can expect excluding candidates using the colors or multiple colors technique.
You can also expect to find X-wing, xy-wing, and swordfish patterns to challenge you. Usually, you will only find one of these patterns in a Sudoku puzzle graded expert.
Pappocom Sudoku wouldn’t create a puzzle that has these patterns. Therefore you can safely conclude their method of grading Sudoku puzzles is different.
Grading Extreme Sudoku Puzzles
Extreme Sudoku puzzles are the most difficult puzzles generated by Simple Sudoku. Not only will you find every pattern mentioned above, but you may find a puzzle that has several swordfish, x-wing, etc. patterns in the same puzzle.
If you are creating your own computer program to generate Sudoku puzzles, then you will count the number of times you encounter each pattern.
If a puzzle has one locked candidate pattern for example, then you would grade the puzzle as standard or medium. If it had multiple locked candidates patterns then you most likely would grade the puzzle as Hard.
Finding a naked triple or quad for sure would upgrade the puzzle grade rating to the next level.
You might assign a mathematical score to each puzzle pattern. Summing the pattern values will give you a number representing the difficulty of the puzzle.
You can set up five ranges of numbers. The score you summed up, will fall into one of the ranges of values. That will be your puzzle grade.
There you have it folks. Grading Sudoku puzzles is just a matter of solving a puzzle, counting the patterns found, and assigning a grade to the puzzle.