Chance only
Solitaires based solely on chance or luck where winning the game depends entirely on the:

• order of the cards dealt; or

• order of the cards drawn; or

• order of the draw; or

• luck of the draw.

Mostly chance
Solitaires based mainly on chance or luck except where an occasional choice of moves will affect the outcome of the game.

Moderate
Solitaires giving moderate opportunity for skill and based equally between chance or luck and skill where making choices is involved half of the time.

Mostly skill
Solitaires giving considerable opportunity for skill where planning ahead is required. Some chance/luck will affect the outcome of the game.

Difficult (games of skill)
Solitaires giving maximum opportunity for skill with very little chance/luck involved. Considerable planning ahead, skillful maneuvering and determination will contribute to the possibility of success.

(The above list represents only the personal opinions of the authors)