The number of samples confiscated with a THC concentration greater than 9 percent has increased significantly, from 3.2 percent in 1993 to 21.5 percent of the 1,635 marijuana samples collected in 2007.
Links
[1] http://www.politifact.com/truth-o-meter/statements/2014/jan/24/patrick-kennedy/has-potency-pot-changed-president-obama-was-high-s/
[2] http://www.rethinkpot.org/tags/potency
[3] http://www.rethinkpot.org/tags/kennedy
[4] http://www.rethinkpot.org/tags/obama
[5] http://www.rethinkpot.org/tags/gmo