There are two methods employed in administering these tests, blind and known. The open proficiency test is like an open book exam. In addition, individual lab workers join associations to beef up their resumes. Tampering Contamination Substitution Tampering refers to allegations that the crime scene or piece of evidence was tampered with. There's a presumption of regularity on the side of the prosecution, so this is hard to prove.
Contamination is easier to prove.
It's just any irregularity in the chain of custody. It mainly involves an attack on individual credentials.