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I am trying to identify features in Sudoku puzzles that separate the "trouble" ones that are hard for me from the ones that are not. But I am having trouble finding techniques to do this.

I have been a fan of solving the standard 9x9 Sudoku puzzles, specially the "Evil" type from http://www.websudoku.com/ . I can solve many but there always seems to be some I cannot solve. I have kept copies of those I cannot solve ( as screenshots) . The number of blank cells at the beginning is the same for all, at 55 out of 81. What kinds of techniques can I use to find common traits in these "trouble" ones? I am thinking factor analysis, but I cannot come up with any factors to start with. I can maybe find a metric on Sudokus ## S_i, S_j ## dealing with the number of filled squares after 10 minutes and set ##d(S_i,S_j)=0 ## if the number of filled squares in each, after 10 minutes is the same , say ## \pm 2 ## (with some adjustments, to allow for the solved ones, etc.) to obtain classes and this may allow me to find some common variables (factors/components) within classes. Any other ideas? Sorry if this is simple, I labeled it as Graduate+ , maybe it is not?

EDIT: Note that 55 blanks out of 81 subsquares is not , by itself, enough to solve a generic Sudoku, but supposedly the ones I am referring to are solvable in the sense that there is a unique solution satisfying the initial conditions.