Q: Why would I trust an industry body like AMTSO to design fair tests?

A: Testing (good or bad) does have an impact on anti-malware product marketing. But the vendors who've joined AMTSO did not do so to prevent people from testing, nor to force them to test only in ways that benefit the industry, but to raise the standard of testing. AMTSO does not seek to define the entire scope of a test: it means establishing an acceptable baseline for such tests.  AMTSO is not designing an anti-malware testing blueprint: AMTSO is saying that a sound test should, at a minimum meet certain, basic criteria. Good testing helps vendors to raise the quality of their products, and that's good for the consumer and therefore for sales. Inadequate and unfair testing can mislead consumers by promoting poor products at the expense of better products, and that's bad for everyone, even those products that gain an unmerited marketing advantage.