“There are known knowns; there are things we know that we know. There are known unknowns; that is to say, there are things that we know we don’t know. But there are also unknown unknowns—there are things we do not know we don’t know." - Donald Rumsfeld
Testing in and of itself is a powerful learning experience. It is a chance for you to demonstrate your knowledge, changes and adds to what you know, and also dramatically improves your ability to retain material, by strengthening and stabilizing the related neural patterns and chunks in your brain.
The testing effect (what scientists unimaginatively call this enhancement in knowledge gained from testing), occurs even if test performance is bad and no feedback is given. Yes, the simple act of taking a test (no matter how poorly you do) makes a difference—truly remarkable. Of course, when you self-test, you’ll want to get immediate feedback, so you can see what you need to focus on and adjust your strategy going forward.
Weaker students tend to grossly overestimate how well they understand material. As a result, they don’t study much; they take the exam and believe they have done really well, and then are stunned when they find out they did poorly (because going in they didn’t know where their gaps in knowledge were).
Don’t fall prey to this trap by simply telling yourself you know the material. Mastering the lecture or text is not the same as the ideas behind them. Quiz yourself (as early and as often as possible) for proof of understanding—otherwise you won’t know the things you don’t know.