Statistical Database Optimization
Consider having a row in the userstat table for each attempt that the user makes at the test. This means, then, that you would have to make the composite primary key of the table to be users_userid and attemptid (which can be an identity column).

I would maybe think that the questions table should be a many-to-many relationship between userstat and questions rather than between users and questions, assuming that each time a user attempts the test they may get different questions. Either way the primary key on this table needs to include more than the single foreign key, it should include both foreign keys.