It seems to me that halves are distributed in a very irregular way. I'm pretty new to this, but in the last month or so I've been through maybe $250 of loose halves and rolls that I found a few at a time. Probably 1/3 of those coins were either 1999-D or 2001-D. Not so many of them in my first box today. Said box had at least 1 1994-P in every roll though, and usually at least five. This despite the fact that only 13-14% of circulating coins in my area come from the Philadelphia mint. Also worth noting is that I've found ZERO 1988-P and 1992-P coins and only one each of 1989-P, 1990-P, 1991-P, and two 1993-P, in all of my searching. All the 1994-Ps in this box were immaculate. After ten rolls or so, I could pick them out by their edges alone!