"Although marijuana grown in the United States was once considered inferior because of a low concentration of THC, advancements in plant selection and cultivation have resulted in higher THC-containing domestic marijuana. In 1974, the average THC content of illicit marijuana was less than one percent. Today most commercial grade marijuana from Mexico/Columbia and domestic outdoor cultivated marijuana has an average THC content of about 4 to 6 percent. Between 1998 and 2002, NIDA-sponsored Marijuana Potency Monitoring System (MPMP) analyzed 4,603 domestic samples. Of those samples, 379 tested over 15 percent THC, 69 samples tested between 20 and 25 percent THC and four samples tested over 25 percent THC."
Lyman, Michael "Practical Drug Enforcement, Third Edition" CRC Press (Boca Raton, FL: 2007), p. 74.