The article as a whole deserves a careful reading by everyone interested not just in the patterns of similarities and differences between the synoptic gospels (and other Christian literature!) but also in integrating those patterns with historically and culturally appropriate models of text-composition. But I leave you with just a small quote from Mattila's conclusion (which builds upon the work of my doctoral supervisor, Loveday Alexander):
Ignoring an essential set of parameters because it is not easy to determine cannot e justified, for such a practice removes source criticism from its concrete historical context into a realm of abstraction that is ultimately historically meaningless. The question of compositional procedures must be addressed so that we can be sure we have grounded the synoptic gospels and related literature, together with those who composed them, in a concrete and historically credible world. (217)
I made exactly this point in Chapters 2 and 4 of Structuring Early Christian Memory (T&T Clark, 2010), though without knowing about Mattila's article. That's unfortunate, for she has stated her case more succinctly and in greater detail than I have.