Therefore tests should be open book iff open book tests are better at measuring understanding than closed book tests.
Are open tests better? I remember from my university days that you can simply look at questions in the book that look sort of similar to the question on the test and apply the necessary substitutions. With open book tests it's remarkably easy to create an answer that looks intelligent about a subject you don't understand at all. With closed book tests this isn't so easy.
Even trivia questions can reveal fundamental gaps in people's understanding. For instance, if a student answers that dinosaurs existed 3000 years ago they must think within a frame of reference where 3000 years is a plausible answer (so no understanding of evolution). So it's a question that is easy to ask, easy to grade, objective, and one that can function as a great litmus test. When all tests are open book you can't ask these questions anymore.
So I'm pretty skeptical about the claim that open book tests are better, period.
As for the argument that people can look things up in books after they graduate, I don't buy that at all. The logical next step would then be to also allow students to ask questions on web forums during exams. Copy question to forum. Get coffee. Copy answer to test paper. After all, when the students get a professional job they may also consult web forums to get their work done.
The reason is that being able to recall at will have a much lower latency than a google search or tabbing through your iphone or android device.
People disparages memorization as mindless, but that's only if you memorize mindlessly about stuff you don't need to know. You can memorize smart, such as your multiplication table.
Our tests hark back to the days when to "look up" something meant going to the library and flipping through an index. Internal recall will remain faster than external look-up for some time yet, but as the balance shifts the current setup will appear increasingly archaic.
For example, human memory follows the law spaced repetition and forgetting curve(our ability to retrieve information exponentially decay). We can take advantage of this by remembering or exercising our memory at the last possible to keep our knowledge. That mean we can efficiently review information as needed. Moreover, as we constantly review information, we only have to repeat them later and later in the future. In pyschology, spaced repetition is one of the least taken advantage discovery. Only in the past two decade or so, did we start using computers to speed up the process of remembering and learning.
That's only by taking advantage of human biology. it's possible in the future that we will enhance the speed and computational ability through bioengineering and neuroprosthetics. More over, the propagation of information is limited by the speed of light. So it's likely those with memory at close at hand will have an advantage over people's whose memory is far away, hence longer latency.
My other memories of open-note tests consist of turning notes in as well. I really liked this idea especially on early exams because the professors could see what you were paying attention to and give you better guidance as to where you shift your focus. Note: I went to a small liberal arts college.
Perhaps it was your substance use or general elevation that lead to your increased performance, not your dedicated effort to memorize the material beforehand?
Though if the test was conducted under a deadline, it stands to reason that you could do better with memorization. As some else noted, the latency is much lower with memorization, but tests do not really mimic the real-world so it is not exactly clear how the lower latency will impact your use of the information outside of a test-like setting.
-- I kinda expect the guy designing a bridge to lookup the correct formulas and not expect that he memorized it correctly. (paraphrased, of course)