This information is brought to us by NIDA (National Institue on Drug Abuse).
NIDA is a governmental organization which researches and sets drug testing standard used throughout government and is usually used to establish private industry standards for drug testing.
This PDF provides a wealth of information regarding testing methodology, detection times, sample tampering, etc...
http://www.nida.nih.gov/pdf/monographs/73.pdf
Many of the examples given are specifically related to marijuana use.
-Hedons