My approach is aimed primarily at people who solve puzzles by human means, and it is easy to understand. If a puzzle is unsolvable by some standard patterns (SP), we will need some kind of chains to solve it, and I suggest that we estimate the complexity of the puzzle according to the following chain classes (CC). I will say that a sudoku belongs to:

- Class SP if standard patterns

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- Class C if we need basic AICs (without group- and ALS-nodes) besides SP in order to solve it;

- Class GC if we need AICs with groups besides the methods of the previous classes in order to solve it;

- Class ALSC if we need AICs with group- and ALS-nodes besides the methods of the previous classes;

- Class FC if we need forcing chains (I accept that they may contain group- and ALS-nodes) besides the methods of the previous classes;

- Class DFC if we need dynamic forcing chains besides the methods of the previous classes in order to solve the puzzle.

I have taken a sample of 10, 318 puzzles from Denis Berthier’s cbg-000 collection and split the puzzles with SER>=7.5 (568 puzzles) into the classes specified above. As I cannot check manually a much greater sample, my analysis may be a starting point for further research in automatic mode.

I have drawn two main conclusions.

1. None of the puzzles of the list needs a dynamic forcing chain to solve it (actually, the sample contains only the puzzles with SER<=9.0), and simpler methods suffice.

2. Forcing chains are rarely needed to solve the puzzles with SER<8.4, and in most cases AICs with group- and ALS-nodes suffice.

Now let us consider the statistical results concerning the puzzles with various SE ratings. It is easy to see that the share of the puzzles of Class FC gradually decreases to 0% as the SER decreases.

SER 9.0: GC - 4.8%, ALSC – 14.3%, FC – 80.9%;

SER 8.9: ALSC – 30.8%, FC – 69.2%;

SER 8.5-8.8: SP – 1.9%, GC – 7.6%, ALSC – 39.6%, FC – 50.9%;

SER 8.4: GC – 29.2%, ALSC – 45.5%, FC – 25.3%;

SER 8.3: SP – 0.4%, C – 3.2%, GC – 48.6%, ALSC – 40.5%, FC – 7.3%;

SER 8.2: C – 3.8%, GC – 96.2%;

SER 7.5-8.0: SP – 9.9%, C – 54.3%, GC - 29.6%, ALSC - 6.2%.

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EDIT

I updated the SER 9.0 statistics thanks to Eleven and Totuan.