First a bit of an administrative note. I am doing fewer posts that primarily highlight what another blogger posts. Want to still get wind of those great things I find out there? Then you can follow me on Twitter, Facebook, or both. That's now my primary means of disseminating worthy work by others.
If you don't want to follow both, all I can tell you at this point is that beyond announcing my blog posts and the good stuff from others that I find, on both, I do a few things differently. Facebook is more for interacting with friends and some family. A decent number of Facebook friends are readers of this blog. Twitter is more for what I call "diarrhea of consciousness." I'm more likely to post mundane updates and photos of what I'm up to on the weekends. Anyway, that's the current method. It will probably evolve. I thought I'd hate Twitter, but I actually don't and have begun to see its value to the purpose for which I am now am beginning to understand.
So, now to the meat of this post, which is an exception to all the above. Very simply, Stephan has an excellent entry on association and causality. It's of critical importance because, for one, you're constantly bombarded by "a new study" and you should know how to regard them critically, and two, because I use epidemiology a lot here. You should make of it only what it is, and no more.
Epidemiology is observational in nature. In other words, investigators gather data passively rather than manipulating variables. For example, if you want to know if people who wear tight shoes develop bunions, you would find a group of people who wear tight shoes and one that doesn't. You would try your best to make sure the groups are the same in every way besides shoe tightness: age, gender, weight, etc. Then you would follow them for 10 years to see how many people in each group develop bunions. You would then know whether or not wearing tight shoes is associated with bunions.
Observational data can never tell us that one thing caused another, only that the two are associated. The tight shoes may not have caused the bunions; they may simply have been associated with a third factor that was the true cause. For example, maybe people who wear tight shoes also tend to eat corn flakes, and corn flakes are the real cause of bunions. Or perhaps bunions actually cause people to wear tight shoes, rather than the reverse. Observational data can't resolve these questions definitively.
Now go read the rest.