  We consider the problem of group testing (pooled testing), first introduced
by Dorfman. For non-adaptive testing strategies, we refer to a non-defective
item as `intruding' if it only appears in positive tests. Such items cause
mis-classification errors in the well-known COMP algorithm, and can make other
algorithms produce an error. It is therefore of interest to understand the
distribution of the number of intruding items. We show that, under Bernoulli
matrix designs, this distribution is well approximated in a variety of senses
by a negative binomial distribution, allowing us to understand the performance
of the two-stage conservative group testing algorithm of Aldridge.
