johnc old chap,afraid you have missed the point. it is the case...

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.