ABC in Roma (#2)

Just a few more words written on my return home from Roma (on an uneventful and sunny trip). On a personal side (that readers can skip!), it was a pleasure as always to be two days in Roma, from running in the early morning, beating the rain and the traffic (and with no map nor camera!) and finding new routes, one on the banks of Tevere and another one along the city walls, to meeting old friends over a plate of pasta, to buying fresh bread and market fruits in the early morning, to enjoying the beauty of La Città Eterna… Too short a trip obviously, but this was/is a busy week!

On the academic side, as mentioned yesterday, the program was quite in tune with my lines of research on ABC and I though a (wee) bit harder about the solutions proposed by the various speakers. One clear tendency is the idea of borrowing from pseudo- or simplified models, either to build estimates (as in indirect inference) or to run the simulation and the calibration (as in, e.g., Olli’s work). As remarked by Judith Rousseau after my talk, we may even have to move further when facing complex models, namely when the simulation of pseudo-samples gets too overwhelming. The issue is then on keeping a connection with the “true” model.

Another theme that crossed several talks is the tension between particle methods (incl. pMCMC) and ABC methods in dynamical models since they usually both apply in such cases. Darren Wilkinson’s talk (that I alas missed in order to catch my plane but recovered by email at the airport, soon to be on-line) did address this opposition, concluding in favour of ABC for the Lotka-Volterra system… My vague feeling is that ABC solutions could indeed come above when they do not rely on the hidden (Markov) structure, in the sense that they do not aim at simulating a joint distribution involving this latent structure…