The highest all-time THC in a marijuana sample was 33.125 from the Oregon State police. This report covers only 2002 May 8 year-to-date. So far, the THC average has shot way up this year. For example, all 46,000+ cannabis samples ever tested at the University of Mississippi have averaged a THC of 3.63%. However, between February 8 and May 8 of 2002. The 1200+ cannabis samples have averaged 5.8% THC.
Marijuana tested there gradually increased in the 3%+ THC range from 1991-1996. Then, from 1997-1999, its THC was in the low 4% + area. The year 2000 saw an average THC for marijuana of 4.69%, and in 2001 it was 5.01%. so far thus year, THC in commercial has averaged 7.79%.
Sinsemilla shows a similar trend. It averaged 6-9% THC per year during 1992-1996. The average was 12-13% in each year 1997-2000, but then dropped to only 9% in 2001. However, so far in 2002 the average THC found in Sinsemilla was 16.09% .That is , the THC in Sinsemilla has close to doubled so far this year versus 2001.

Source: University of Mississippi (Marijuana) potency Report 2002