This commentary is nicely laid out, I think. Pip, maybe this answers some of your questions in the other research thread?
http://www.revolutionhealth.com/blogs/valjonesmd/false-positive-resear-9991
"And thirdly, the scientific method (if not carefully followed in
rigorous, randomized, placebo-controlled trials) is a set up for false
positive tests. What does that mean? It means that the default for your
average research study (before it even begins) is that there will be a
positive association between intervention and outcome. So I could do a
trial on, say, the potential therapeutic use of candy bars for the
treatment of eczema, and it's likely (if I'm not a careful scientist)
that the outcome will show a positive correlation between the two."