Testing as learning, not just measuring
The traditional view separates learning (studying) from testing (measuring). A student studies by reading, then is tested to see how much they learned. But research shows that retrieval practice, the act of pulling information from memory, is one of the most powerful learning techniques available.
In one landmark study, students who spent study time taking practice tests recalled 50% more on a final test than students who spent the same time re-reading. The act of retrieving the answer strengthens the memory more than passively seeing the answer again. Pulling information out of memory is harder than putting it in, but the effort pays dividends in retention.
Frequency and timing of retrieval
Not all retrieval is equal. Testing once on a topic provides some benefit, but testing multiple times spaced over time produces much better long-term retention. A quiz on Day 1, again on Day 5, and again on Day 20 produces better retention than three quizzes on the same day.
The difficulty of retrieval also matters. A cued recall test (fill-in-the-blank with a hint) is less effective than free recall (recall with no cue), which is less effective than inference and transfer tasks (apply the concept to new situations). The harder the retrieval task, the stronger the memory that results. This is why open-book exams and searchable materials do not eliminate the benefit of retrieval practice; retrieving from memory is fundamentally different from retrieving from an external source.