m_b_metcalf wrote:... but Sudoku Explainer is usually used in this Forum as a reference.

OK thanks

I've just downloaded Sudoku Explainer.

A puzzle that scores 2182 with HoDoKu scores 7.1 with Sudoku Explainer.

I suppose Sudoku Explainer goes about it the same way, deciding how difficult it is by which methods are used.

EDIT

It's in the FAQ for Sudoku Explainer...

"How does the Sudoku Explainer rate the difficulty of a Sudoku?

The difficulty of a Sudoku is rated by the hardest solving technique that is required to solve it. This is not necessary the best rating scheme, and many other solvers might use different rating for some solving techniques (see previous question).

More precisely, the Sudoku Explainer uses the following difficulty ratings of the solving techniques:

1.0: Last value in block, row or column

1.2: Hidden Single in block

1.5: Hidden Single in row or column

1.7: Direct Pointing

1.9: Direct Claiming

2.0: Direct Hidden Pair

2.3: Naked Single

2.5: Direct Hidden Triplet

2.6: Pointing

2.8: Claiming

3.0, 3.2, 3.4: Naked Pair, X-Wing, Hidden Pair

3.6, 3.8, 4.0: Naked Triplet, Swordfish, Hidden Triplet

4.2, 4.4: XY-Wing, XYZ-Wing

4.5 - 5.0: Unique rectangles and loops

5.0, 5.2, 5.4: Naked Quad, Jellyfish, Hidden Quad

5.6 - 6.0: Bivalue Universal Graves

6.2: Aligned Pair Exclusion

6.5 - 7.5: Bidirectioal X-Cycles and Y-Cycles

6.6 - 7.6: Forcing X-Chains

7.0 - 8.0: Forcing Chains, Bidirectional Cycles

7.5 - 8.5: Nishio

8.0 - 9.0: Cell/Region Forcing Chains

8.5 - 9.5: Dynamic Forcing Chains

9.0 - 10.0: Dynamic Forcing Chains (+)

> 9.5: Nested Forcing Chains

It seems that the difficulties 1.2, 1.5, 1.7 and >= 2 correspond to the degrees 1, 2, 3 and 4 of some newspapers. Note that many solvers are rating Naked Singles easier than Hidden Singles (which is reasonable when candidates are always visible). According to some sources, "diabolical" Sudokus are those that are not solvable without trial and error. But there is a great controversy on what "trial and error" means.

The Sudoku generator uses the following levels:

Easy: difficulty 1.0 to 1.2 (can be solved using Hidden Singles in blocks only)

Medium: difficulty 1.5 (requires Hidden Singles in rows or columns)

Hard: difficulty 1.7 to 2.5 (can be solved without writing down candidates)

Fiendish: difficulty 2.6 to 6.0 (can only be solved by writing down candidates, but does not require Forcing Chains)

Diabolical: difficulty 6.2 or more (can only be solved with Forcing Chains)"