I have been reading about QFT amplitudes. It seems that difficulty increases as we consider more and more terms in the Dyson's expansion for the Scattering operator, and we need to normalize each of them if we want to get a sensible result. My question is, nature usually uses what order? I mean, what type of Feynman diagrams "is most used by nature"? I imagine that she randomly uses any order for a same type of interaction, at each time. So my question would be better posed as, what type of Feynman diagram occurs with more frequency?