Which machine learning algorithm should I apply for differentiate question difficulty level with users' result Here's the scenario, There's a database with thousands of single-option questions for testing a specific skills, and a large number of users (either professional or amateur in this skill), each of which answer 10 random questions from the database.

The only thing that I can think of is to differentiate question difficulty level according to the correct rate of each question. But how could I take fully use of other informations like:


the correct rate from the user's perspective and feedback to its own influence to the difficulty level of questions (if user A answers 9 out of 10 questions correct, then the incorrect one (question_10) is more likely to be harder, than an user B answers 1 out of 10 questions correct and question_10 is in the incorrect set)
the answer time for each question by each user


Could anyone give me some ideas on this model, like where should I delve more into to make the difficulty level of question more accurate? Great thanks!
I suggest you look at item response theory and the Rasch model.  They construct a model that attempts to identify both (a) the proficiency/knowledge/ability of the user and (b) the level of difficulty of the question.