I know I can give some examples. Let's say I am interested in quizzes on dogs.
One quiz can be difficult because it's got genuinely difficult but interesting questions, such as "Once the darling of both gamekeepers and the English aristocracy, this handsome breed is now quite rare. However, it is as versatile as ever- no other sporting breed comes close to it in the number of titles earned per dog." with the choices of Nova Scotia duck tolling retriever, flat-coated retriever, Chesapeake bay retriever and curly-coated retriever.
The next quiz could be difficult because it has questions like "What is the average weight of a female Great Dane?" with the choices of 100, 110, 120 and 130 pounds.
I know that I would rate a quiz with questions like the first quite well, and questions like the second quite poorly.
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"I'd rather make one dog happy than please all the dogmatists in the world."
P. Z. Myers