A Dowsing protocol

[StewartP]

I have someone who is willing to test his claim to be able to dowse metal with rods.

I have proposed a basic protocol to him of 20 bowes, 10 conatin bags of sand with metal in, 10 just bags of sand

If he gets more than 12 out of 20 right I've pldeged 100 dollars to a charity of his choice.

I'd like some guidance to tighten the protocol to make it fair and foolproof.

He hasn't yet specified which "metal" he claims to be able to dowse. I'd like to avoid iron in case of concealed magnets.

Any other ideas?

Thanks

stew

[jimtron]

I remember reading about a dowsing test here, where a dry run was done first. For the dry run mark the bags containing metal, and have the dowser make sure his "rods" are working. That way they can't complain that there was bad vibes or interference or something.

[Olowkow]

If you introduce the money incentive, ($100), it might invalidate the results. :)

[rjh01]

I would also suggest he needs to get more than 12 / 20 right. Do the maths. He stands a good chance of getting that by chance alone.

[StewartP]

I would also suggest he needs to get more than 12 / 20 right. Do the maths. He stands a good chance of getting that by chance alone.

This is the sort of info I need, I don't want to be let down by poor mathematics

I would have thought that with 20 bags, 10 with metal, 10 without, chance would give him 50/50 so by asking for 60/40 I was already eliminating chance?

[rjh01]

Not sure of the exact odds but giving him a 40% chance means that if you do the test 10 times you can expect to lose 4 times. The one test you do could be one of those 4 times.

[Andrew Wiggin]

Ask him what level of confidence he has in his powers. Hopefully he'll make it simple, and say he can do what he thinks he can do one hundred percent of the time. Then just make it 20 out of 20. I bet he can do that with no problem, when he knows where the targets are. Once the test in blinded, if it's a real power, it should work to the same extent it does when open.

A

[EHocking]

This is the sort of info I need, I don't want to be let down by poor mathematics

I would have thought that with 20 bags, 10 with metal, 10 without, chance would give him 50/50 so by asking for 60/40 I was already eliminating chance?Others will be bored with me posting this link, but this is what I use to quickly check probability of random chance without having to do the maths.

http://www.automeasure.com/chance.html

From this we get, for 1/2 chance of success and 20 trials, success due to random chance alone is expected to be:

At 1:100 - 4 to 16
At 1:10,000 - 2 to 18

[skyhand]

Have him do it 3 times. Two times with no metal in any of the bags. This would make it a little harder.

[blutoski]

I'd ask the dowser to help you with input, too.

The purpose is to head off post-hoc rationalizations.

Just as an example, I expect that with the protocol you have in mind, he may say that the metal in a bunch of bags scattered about would make it difficult. Or that metal in the building structure sends a stronger signal than the bag's contents. Or whatever. (This is also the advantage of running a bunch of unblinded dry runs, as mentioned above)

He may prefer to look for just one metal bag at a time, in a space with little or no metal in an arbitrarily close proximity. This probably more closely approximates what he is asked to do 'in the wild'.

You could use exactly the same binary stats and run randomly sorted pairs 20 times. The task would be that he just has to tell you which one of the two bags contains the metal, twenty times over.



You could also do the 'one metal target' by burying 20 containers in sand at the beach, one of which has metal in it. Flag them all to eliminate ambiguity, and he just has to eventually tell you which flag is above the target (metal-filled) container. This approximates p<=.05. If you want p<=.01, bury 100 containers.

[Lanzy]

I haven't read how you intend to blind this. How many confederates do each of you have? How do you intend to choose which bag to put metal in? how far apart will the bags be? Will the bags weigh the same with and without metal? how much metal, can you just load it with a few bb's?