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Prior Probability

Entry 1042, on 2009-06-26 at 20:17:37 (Rating 3, Skepticism)

A common comment I hear in skepticism discussions is that extraordinary claims require extraordinary evidence. This means that if someone is making a claim contrary to what is considered to be well established science then they need to produce more proof than they would for a less controversial claim.

I have always been a bit unsure about this idea. Why should new ideas require more proof than well established ones? Doesn't that make science biased against new ideas? After listening to another Skeptics' Guide to the Universe podcast I think I now have a new perspective on the idea and it isn't really a source of bias after all.

It gets back to the idea of prior probability. The thing is that you have to consider the total evidence for an idea. If a new idea fits in with all the existing evidence then it already has a head start. If it contradicts what we already know then it has to overcome all of that negative evidence before it even enters the realm of positive support.

As an example consider homeopathy. That is the belief that samples of substances which cause a disease when diluted to unbelievably low concentrations can actually treat the same disease. There are a few problems though. First, the dilutions are so great that there is none of the original substance left at all, its just water (or some sort of filler in solid homeopathic remedies). Second, why should something which makes the condition worse in normal concentrations make it better when its diluted?

There have been studies which seem to support homeopathy but most (maybe all) have been found to be deficient in various ways. One of the most famous was shown to be caused by a researcher at the lab deliberately changing the results to suit the outcome they wanted.

If there were a similar number of results supporting a theory which fitted in with what we already know then we could take it more seriously, but having conflicting results, the positive results coming from poorly designed experiments, and having no prior probability means that homeopathy is just not believable.

One alternative explanation for the observed phenomena is the placebo effect. There is plenty of evidence this really happens and it fits with what we already know so that seems like a much better explanation of unusual claims like homeopathy than the alternative explanation of water remembering the "vibrations" of previously dissolved substances.

Prior probability is weak for many forms of pseudoscience, superstition, and the paranormal, for example psychic abilities, UFOs, healing prayer, near death experiences, creationism, astrology, acupuncture, Nostradamus, numerology, reincarnation, flat Earth, the Loch Ness monster and bigfoot.

Some of those are more likely (or less unlikely) than others and I'm sure there are many I've missed out but as more scientific evidence accumulates which contradicts those ideas they become less and less likely and the evidence to prove them becomes greater. So I think the idea that "extraordinary claims require extraordinary evidence" makes sense after all.

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