





The Relative Incidence of Sudoku StrategiesThis article has been updated December 2013 and replaces the statistics done in Dec 2009 and March 2012 (you can compare the previous data on that page). A recent question from a reader prompted me to run off some statistics which I think are interesting and worth exploring. Comment:There is something I am curious about that I really hope you can answer, although it's quite subjective and I suppose the answer will be a ballpark figure but I was hoping a Sudoku expert such as yourself could take your best educated guess at. If I know all of your basic, tough, and diabolical strategies, but don't go as far as any of your evil strategies that you list, what percentage of Sudoku puzzles (in your opinion) do you think I could solve80% of all puzzles that I would try? 85%? 90%? 95%? 99%? What would you guess if you had to estimate? I know it's hard since there are literally trillions of puzzles, but easy, medium, tough, and many diabolical puzzles I can already solve with these current strategies, excluding your evil ones. Do you think the percentage of puzzles where you HAVE to use one or more evil strategies in order to solve the puzzle is a small percentage, perhaps 1%? 2%? 5%? 10%? Just curious what your opinion is. There is a lot to grading and scoring a Sudoku puzzle. I've put some thoughts about this into http://www.scanraid.com/Sudoku_Creation_and_Grading.pdf. There is not a one to one correspondence between the published grade (or the grade on my solver) and the list of strategies and many factors contribute to the grade. My strategy list is partially subjective in that I choose to label certain strategies as 'tough' for ease of explanation and to show what I consider the best order in which to attack a puzzle. It is an attempt at a 'minimum path'. It should also be noted that because I don't use strategy X to solve a puzzle in the solver, it does not follow that strategy X could not be used. There are often many ways to solve the same puzzle. However it is still an interesting question what proportion of all puzzles require at least one strategy in each grade group. I've run a count on a 120,000 puzzles I created searching for unsolvables (December 2013). These were produced randomly and I did not know the grade until after I created them. The sample is therefore fair. The results are:
This confirms my view that the vast majority of puzzles are uninteresting. In order to produce a 100 puzzles of all grades I need to over produce many puzzles since the incidence of higher grade puzzles is low. Note that the 10% of 'moderate' only puzzles does not mean they are rare. Any hard puzzle will require many more incidences of moderate strategies to complete in addition to the hard ones. It follows that I can produce a list of all the Sudoku strategies and a count of their occurrences in solving the stock. Where different types or rules are available I've also added those as seperate figures. *Note: The strategy count (larger white numbers) counts puzzles where the strategy is used, not how many times. The bluish smaller numbers for DO count the number of times the substrategy is used.
So if you were wondering, as I was, how useful certain strategies are, this data is interesting. The only other caveat I'd add is that some strategies are subsets of others, or can be expressed in terms of another strategy. For example, Remote Pairs are a special case of XYChains which is a subset of AICs. It is useful for the solver to split these out but when making and grading I don't do so. So there is some overlap. The answer to the reader's original question  the incidence of 'evil' strategies, is I'd say, about 5%. Andrew Stuart 

Comments
Comments TalkMonday 24Dec2012
... by: Fred
Very, very interesting ! You have all my compliments and my deepest admiration. Unfortunately I am not of english mother tongue, so for me difficulties always grow harder and harder.Do you think I can find anything on this subject printed in ITALIAN ? eventually, where ?
Thanks for the attention,
Emmanuele Frediani (from Italy)
P. S. : Just a curiosity : How old are you ?
Friday 29Jun2012
... by: slowofmind
Smashblast, could you please say what year is the Scientific American article "The Science behind Sudoku".Tuesday 1May2012
... by: keith
Hmmm. It would be interesting, perhaps, to mix up the order of your solving strategies, and try a series of them, looking for 1) how they change the incidence of use, and 2) whether/how some orders reduce the length of puzzle solving. In the second instance, one can imagine that puzzles "requiring" a diabolical strategy, for example, might benefit from it being applied earlier... or not.There are a great many orderings you could try, I'm not sure which would be most interesting/powerful, if indeed any succeed over the default...
The ordering I have settled on is subjective in a certain sense because I've decided on it based on my hunch that strategy A is more complex that strategy B. The early strategies I think are no brainers but diabolical and above are more difficult to order.
I would like to allow the user to order the strategies but I don't know how to do this without greatly complicating the interface  another trade off
Wednesday 8Feb2012
... by: Smashblast
There was an eneeellcxt article on the "science behind Sudoku" in July issue of Scientific American, with more strategies, worth reading.Monday 20Sep2010
... by: joseph brophy
thank you so much for your contributions to sudoku. i am preparing to teach a class to seniors, and this is the type of information that sheds light on the game. jtb