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ANC Workshop: George Papamakarios and David Sterratt, Chair: Aleksej Stolicyn

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What
  • ANC Workshop Talk
When Nov 21, 2017
from 11:00 AM to 12:00 PM
Where IF 4.31/4.33
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George Papamakarios

Title:
Masked Autoregressive Flow for Density Estimation

Abstract:
Autoregressive models are among the best performing neural density estimators. We describe an approach for increasing the flexibility of an autoregressive model, based on modelling the random numbers that the model uses internally when generating data. By constructing a stack of autoregressive models, each modelling the random numbers of the next model in the stack, we obtain a type of normalizing flow suitable for density estimation, which we call Masked Autoregressive Flow. This type of flow is closely related to Inverse Autoregressive Flow and is a generalization of Real NVP. Masked Autoregressive Flow achieves state-of-the-art performance in a range of general-purpose density estimation tasks.

 

 

David Sterratt

I hope to give a double-bill arising from two MSc projects.

1. A problem when inferring parameters of dynamic systems with noisy starting conditions

(work with Judy Borowski)

To infer the dynamics of biochemical reactions involving divalent cations, such as calcium, solutions are prepared containing the reactants and photolabile chelators, such as DM-nitrophen, which "cage" the cation. A flash of UV light cleaves the chelator, uncaging the calcium, thus allowing the reactions to proceed. The progress of the reactions is measured by a calcium-sensitive fluorescent dye. The experiments are repeated under multiple conditions by varying the concentrations of the reactants and changing the length of the light flash to control the amount of calcium uncaged.

The resulting set of fluorescence time-courses and known parameters (concentrations and known binding coefficients) is used to infer the unknown parameters in a model of the studied reactant's binding to calcium. In order to do this the entire system of reactants, including the photolabile chelator and fluorescent dye, is modelled as as a set of ordinary differential equations. The maximum likelihood estimate of the unknown parameters is found by minimising the goodness-of-fit over all conditions between the experimental fluorescence and the fluorescence derived from the model. The Hessian of the goodness-of-fit with respect to the parameters indicates the distribution of unknown parameters.

One problem faced is that the starting conditions are noisy: the length of the light flash predicts the amount of calcium, but with considerable noise around the estimate. If more calcium is released than predicted, the whole time-course will be shifted, meaning that on average the simulated traces are poor fits to the data.

 

2. A proportional electoral system with single member constituencies

(work with Aisling Mac Ardle)

 Due to the first past the post (FPTP) electoral system, general election results in the United Kingdom are disproportionate to the level of national support held by political parties. In contrast, proportional electoral systems, such as single transferable vote (STV) and party list systems, provide proportional results but require more complex voting. With the goals of (i) producing election results that reflect all votes cast by the electorate, (ii) having single member constituencies and (iii) maintaining a simple voting process, as in FPTP, we have developed a novel electoral system, the Concentrated Vote (CV). CV moves votes between constituencies from candidates with low vote shares to candidates from the same party with a higher vote share. We apply CV to the results of UK general elections from 1979 to 2017, showing that CV improves proportionality over FPTP. Voter acceptance of CV might depend on the geographical unit to which it is applied.