Tracking multiple targets is a challenging problem, especially when the tar
gets are "identical", in the sense that the same model is used to describe
each target. In this case, simply instantiating several independent 1-body
trackers is not an adequate solution, because the independent trackers tend
to coalesce onto the best-fitting target. This paper presents an observati
on density for tracking which solves this problem by exhibiting a probabili
stic exclusion principle. Exclusion arises naturally from a systematic deri
vation of the observation density, without relying on heuristics. Another i
mportant contribution of the paper is the presentation of partitioned sampl
ing, a new sampling method for multiple object tracking. Partitioned sampli
ng avoids the high computational load associated with fully coupled tracker
s, while retaining the desirable properties of coupling.