A TPR instructor once told me that MCAT diag scores typically decrease before they increase. That's not to say that you will score below your first diag, but.. I'd say it's perfectly normal for your scores to go: 20 (taking it cold) 24, 22, 23, 27, 30, 29, 32, etc.
Remember, every test is asking different questions. If you get a good test that you are very familiar with, you are more than likely going to score high. If you get something that is still convoluted in your head.. you'll remain in the mid to low 20s.
Just pray that you get VERY familiar test subjects on the real thing. I took it once and got basically ZERO physiology (which is my bread and butter as a physiology undergrad). All genetics and immunology.. I struggled and got a 9 on BS when I was getting 11s and 12s on most of the diags. It's a crapshoot I tell ya!! Just buff up on the weak stuff!!
The same will go for the PS section.. as for verbal, practice makes perfect!
