[uglycoyote]

Yes I would think that you are dealing with sparse matrices with little clusters of nonzero values in a model like this.

One could say that I'm "part of a terrorist matrix", and that my value in the matrix is zero. We are all part of a terrorist matrix. I hope the CIA is not reading this, because they probably are not mathy enough to know what I'm talking about, and they may just take my quoted statement at face value, but in a mathematical sense we probably all actually have some nonzero value in our rows and columns of the terrorist matrix. You know someone, who knows someone, who knows someone, who is a terrorist. It really just depends how many people and how many degrees of separation you want to include in your matrix.