While we look at using the same hood and exposing it all to the same conditions as a control, the environment poses an endless combinations of factors affecting the results. Like Scott, I live in an area where this incredibly low amounts of pollution, no acid rain, no fallout from industry or railroads, etc. As such I can get my coat of P21S to last longer than most people just because of the environment. Anyways, I think it is great to have more relatively empirical studies regarding these waxes and hopefully we will begin to reach some sort of precision between these tests.