How many questions do we have to get right to get a 230??? Does that mean that we answer 230 questions correctly or is it some sort of percentage?
no.
each question has it's own performance profile, and the test is normalized to those profiles for each test. therefore, if you get a 'harder' test (objectively, accord to the aggregate performance profile of the test) you can get more wrong than someone with an objectively 'easier' test. the variation between the extremes of the aggregate performance profile isn't known to outsiders. the %correct/score correlation is almost certainly non-linear-especially at the extremes-though in some parts of the curve it may look linear.
there are not exactly 50 experimental questions. there is variance in the number you can get, with 50 probably being the average number of 'experimental' questions that are sampled out of the large original pool of questions (probably ~5000-10000 questions, but just a wide-ranged guess, probably not more than that). your whole test is sampled randomly from the original pool, so on average you should get ~50.
the 'experimental' questions are most likely not simply 'wierd tester questions', but questions they want to accrue more 'performance data' on before 'releasing' them as a regular question. if 'experimental' questions have a dismal performance profile, being either too easy or hard according to whatever variance cut-off they have, they will probably be screened out reasonably quickly.
they only use 1 years worth of performance data to calculate your score, so it doesn't matter when you take the test, and you are normalized to what is approximately your cohort.
