probabilities, page-10

johnc old chap,
afraid you have missed the point. it is the case that each well may have in itself a 10% pos, however we are talking here about the a priori chance of success in not one well but in my example a set of 3 wells, each of which is an independent test (which may be anywhere on god's earth) with a pos of 10%. before any well is drilled the chance of success in the campaign is 1 minus (.9 times .9 times .9) equals 27%. but this is an estimate for the drilling campaign, obviously not for a single well. once a well is drilled its pos is known, namely 0% or 100%, and the campaign reduces to 2 wells which considered as a group then has a success chance of 19% i.e 1 minus .9 times .9). when the second well is drilled again its pos becomes known and the campaign reduces to a single well which has the chance of success of 1 minus .9 equals 10%, which is precisely what one expects.
similarly for the coin tossing example of another poster. although the pos for a heads is .5 (50%) for a single toss, if you consider tossing 4 times then you have a chance of 93.75% of getting one or more. of course you may still not get one and that is where the other 6.25% comes in. aint statistics grand.