We consider the following urn model. At the beginning of trials the urn contains a given number of balls of N colours. In each trial, a ball is randomly drawn from the urn, independently of the results of the other trials. If we draw a ball of some colour, then before the next trial the number of balls of this colour in the urn is changed according to a certain rule. Before the trials, a level Vj is determined for each colour j, j = 1, . . . , N\ we assume that v\ , . . . , z/yy are independent integer-valued random variables. The drawings stop when the numbers of balls of arbitrary k colours attain or exceed the corresponding levels for the first time. Decomposable statistics (DS) are studied, where g\ , . . . , g N are functions of the integer argument and η\ , . . . , η Ν are the numbers of balls of corresponding colours at the stopping time. The characteristic functions of DS Z/w* are obtained by embedding the urn scheme in a suitable Markov pure birth process. Some special cases including sampling with and without replacement and Polya sampling are considered.