In the cowboy movies, the last gunslinger standing rides off into the sunset with the girl.  In online advertising its often the same; the last ad shown gets credit for the conversion.

In a recent iMedia article on ROI Young-Bean Song from Atlas pokes some worthy holes in that simplistic view of the world.  I want to extend the argument to include all media.   We know TV affects search and radio affects mobile.   A full attribution model would look at the entire integrated media plan.