[Nets-seminars] First UCL EE talk of 2014
Richard G. Clegg
richard at richardclegg.org
Fri Jan 10 15:57:20 GMT 2014
Our first talk this year is by Dr Damien Fay of the University of
Bournemouth. It is at 16:00 on Friday 17th January and is in 66-72 Gower
Street room GS/102.
Approximate Bayesian Computation and Complex Network Models
The aim of this talk is to introduce a method called ABC which can be
used to estimate parameters in models where the usual estimation
techniques do not apply. In addition, we introduce ABC into the are of
graph generator tuning and see that while it can be used there are
several issues that need to be addressed.
This talk is broken into two sections. The first is somewhat a tutorial
which focuses on ABC and gives an introduction to what we mean by
complex processes and no clear means to estimate the parameters of that
process (i.e. the blurb below will be explained in the talk). In the
general area of model fitting, likelihood/Bayesian based approaches are
popular not just because of the excellent parameter estimates but also
because they return the posterior distribution of the parameter – i.e.
we can see how probable it is that a parameter actually has a different
value from the one we are using. One can think for example of the case
where the posterior is almost uniform which essentially means the
parameter estimate is useless; in this case care should be taken when
deriving conclusions from the model.
Applied Bayesian Computation (ABC) is a method for estimating the
posterior distribution for model parameters when the a likelihood based
approach doesn't work; typically this occurs when the likelihood
function a) doesn't exist, b) the likelihood is vanishingly small or c)
it is just too complicated or time consuming to derive (especially if
one is not convinced the model is appropriate in the first case). ABC
has been around for many years but has recently become very popular due
to some advances in the “summary statistic selection problem” - the
Achilles heel of ABC.
The second half of the talk looks at graph topology generators. It may
come as a surprise that 11 years after the introduction of the
preferential attachment model there still exists no likelihood
expression for that model. For the small world model one can only place
bounds on the parameters. For models that include anything of greater
complexity one is left without any option. This research thus asks the
question, can ABC be applied to any graph topology generator? We then
look at the answer and uncover some interesting technical issues leading
to a set of heuristics for using ABC. We also take some real world data
sets and show how using ABC we can select appropriate models for those
datasets.
Bio: Damien Fay obtained a B.Eng from UCD (1995), an MEng (1997) and PhD
(2003) from DCU and worked as a mathematics lecturer at the National
University of Ireland (2003-2007) before joining the NetOS group,
Computer Laboratory, Cambridge from 2007-2010 as a research associate.
He was a research fellow at the Cork centre for computational
complexity, 4C, University College Cork from 2010-2012. Damien recently
took up a tenured lectureship in big data analysis with the smart
technologies research centre at Bournemouth university. Damien is best
known for developing the wavelet transfer model in time series analysis,
the weighted spectral distribution metric in applied graph theory and
recently lodged a patent for an arrival time prediction algorithm which
has won a UCC commercialisation 2012 award. His interests are in the
areas of time series analysis and applied graph theory.
--
Richard G. Clegg,
Dept of Elec. Eng.,
University College London
http://www.richardclegg.org/
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