From my experience of searching through old SDN forums, some say our scores are compared to first time test takers from 1998 to 2003. Some say our scores are compared to scores from the previous test cycle. Honestly, no one really knows which group we are compared against or how our composite and raw scores are compiled.
What we do know is one of the two essays is graded and the other is experimental. As stated by mug3n, 8 out of the 48 questions are experimental and only 40 questions are counted in each of the 5 subsections.
I'm not sure if anyone who has taken the PCAT multiple times have noticed that percentile rank composite scores over 50% gets inflated and composite scores under 50% gets deflated.
Lets take two sets of scores:
Example #1)
Verbal
439 94
Biology
450 97
Reading
452 98
Quant
426 87
Chem
415 75
Composite
436 96
The average percentile rank score for 5 sections = (94+97+98+87+75)/5 =
90.2
yet the percentile rank composite =
96
Note that the average of the 5 scaled scores equal the scaled composite score but this is not the case for composite scores. (e.g. (439 + 450 + 452 + 426 + 415)/5 = 436.4)
Example #2)
Verbal
403 56
Biology
387 32
Reading
404 56
Quant
409 68
Chem
384 27
Composite
397 45
The average percentile rank score for 5 sections = (56+32+56+68+27)/5 =
47.8
yet the percentile rank composite =
45
Note that the average of the 5 scaled scores equal the scaled composite score but this is not the case for composite scores. (e.g. (403 + 387 + 404 + 409 + 384)/5 = 397.4)
A common analogy for the above composite score relationship is "The rich gets richer and the poor gets poorer"
If you check people's PCAT scores from the following forum,
http://forums.studentdoctor.net/showthread.php?t=427578
you will notice the above relationship holds true for all PCAT composite scores.
P.s. If composite score is exactly at 50% or slightly above 50%, then the average percentile rank score for 5 sections equal the percentile rank composite.