An old MCAT score in the 75th percentile should NOT be equated to the 75th percentile of the new MCAT. The new MCAT is not only longer, but it has a lot more information on which it tests. You therefore can't compare students that took different versions of the MCAT by simply looking at percentiles.
I understand your concern. Let's play with this a bit.
First of all, the point of percentiles is to gauge where applicants stand, relative to their peers. They don't have to necessarily compare new vs old, but as
@gettheleadout stated, both pools of test takers face very similar/same tasks and conditions. In fact, people still fluctuate in scores on the
current MCAT, simply due to differences in the passages they receive (ex: I love economics and cardio, but get 3 passages each on Monet and hemorrhoids). You can't really do much for this. They assume that the test forms (old) do not statistically differ from one another, and that the large pool allows for a nice, normal spread with which to gauge applicants.
Now: regardless of what you think about the difficulty of
either exam, you gauge whether an applicant did well compared to peers that took the same exam, if you have doubts. If they are purportedly the same, then they may cross-compare. Adcoms (and everybody, for that matter) have virtually zero data to work with, besides the percentiles being given to them just now with the scores.
It's not perfect, and you can argue a lot of things. But adcoms aren't dumb. You can't argue that you were gypped if your (although lower) 87th percentile score was beaten by somebody with an old MCAT of 36 (~96th percentile for 2014), because "my new test was harder". They have no real data regarding difficulty. AAMC says that yes, the new MCAT is longer, but due to the large number of questions, it also gives a more fair, accurate assessment. Who knows?
Okay, so, is a 75th percentile on the old MCAT
not comparable to a 75th on the new MCAT? Look at it this way: it likely
is; Both exams largely overlap in terms of skills, and you could assume that, since the new one STILL tests the old skills/materials, but with additional stuff, that a large chunk of the old MCAT is represented in the new one, and it is therefore not very different for the most part. If that's the case, then you could argue that you can cross compare.(***You also might wish to assume that the pool of test-takers doesn't change much from year to year, and that with normal preparation, the difference between old and new MCAT percentiles of each individual,
had they taken both, may not have actually changed much at all. No data to support it, but not an unreasonable thought.***)
Either way, a person who scores in the upper percentile on
either exam is going to be looked at nicely by adcoms. They don't have to do regressions or comparisons between exams, because you're already being compared to a large group of peers within-group, who is assumed to be roughly similar to previous cycles, taking one of two exams that test an extremely large amount of overlapping skills similar to the old MCAT. You're not suddenly going to drop to the 10th percentile, and in admissions, adcoms don't sit there and say, "Hmmm..what a great applicant, but the other one is 3 percentile higher, and since everything is even, let's guess that this MCAT is easier, and we'll take the other one!" It's not black and white.
Keep in mind, I'm simply pointing out factors to take into consideration.
Gahh,
@justadream beat me to it for the most part.