So, let's talk about double blind tests for a moment.
This is one of those things that a lot of people have trouble understanding. Even people who remember how the scientific method works often don't understand what double blind tests are, or how and why they're used.
First off, let's talk about what double blinds are. At the most basic, a double blind experiment is one where neither the subjects of the experiment, nor the ones directly administering the experiment, know the exact details of the experiment. So, while someone sets up the experiment, and knows what each group in the experiment is doing, they don't interact with the subjects.
Now, they're not used for every experiment. You'll notice I mentioned "subjects." Double blinds are typically used when experimenting on people, or when people can affect the results of the experiment without realizing it. How can that happen? And how can double blinds stop it from happening?
Drug trials are a good example of this sort of experiment. Just being told they're taking a medicine can help people feel better, which is what we call the placebo effect. Because of that, it can be difficult to tell if a drug's actually had an effect, or if the person just felt better for thinking it was.
Similarly, tests for psychic abilities (like dowsing or reading minds) can be affected. Obviously, if the subject knows where the water or gold is hidden beforehand, they're going to focus on that object. Even if they're not consciously aware that they're doing so, they can still do it without realizing it.
And moreover, perceptive subjects can pick up on the reactions of those administering the tests. In the drug trials, they might be able to tell if they're really being given a drug or not. In a test of psychic abilities, the psychic might pick it up from the faces of those testing them, seeing if they're tensing up as they get near the object.
So, we withhold that information from everyone present at the test. This is why it's called a "double" blind, as opposed to a single blind, when only subjects are kept in the dark. Even the ones who are directly administering the test aren't aware of who's in the control group, or where things are. That way, the drug and the placebo are competing on equal ground, and the drug has to do significantly better than the placebo to be declared effective. The psychic has to beat random chance with no clues available before he can be taken seriously.
Double blinds are there to reduce as many extraneous factors from the experiment as possible, so that only the factor you're testing for--drug efficacy, dowsing, or dog food taste--is actually being tested. A double blind is much more accurate, and much harder to argue with than a single blind or no blind test.
