The samples included cannabis, hashish, and hash oil. The cannabis samples were further subdivided into marijuana (loose material, kilobricks, and buds) and sinsemilla, Thai sticks, and ditchweed. More than 82 percent of all confiscated samples were in the marijuana category for every year except 1980 (61 percent) and 1981(75 percent). The concentration of THC increased from less than 1.5 percent in 1980 to approximately 3.3 percent in 1983 and 1984 and then fluctuated around 3 percent until 1992. The potency of confiscated marijuana samples has risen continuously since 1992 and has gone from 3.1 percent in 1992 to 4.2 percent in 1997. The average concentration of THC in all cannabis samples rose gradually from 3 percent in 1991 to 4.47 in 1997. In contrast, hashish and hash oil had no specific potency trends. Other major cannabinoids also had no significant change in their concentration over the years. Tables, figures, and 19 references (Author abstract modified)