Given a number (k) of geological classes, how can they be optimally assembled into two groups, so that the first group represents “Rn prone geologies” and the second group “non-Rn prone geologies”, according to a given criterion. A “Rn prone geology” (RPG) is one for which a Rn related quantity shows higher levels than for “non-Rn prone geology”. Among such quantities arethe mean indoor Rn level, the probability that indoor Rn concentration
exceeds a threshold or reference level, or the RP.