Posterior analysis of random taste coefficients in air travel behaviour modelling

Increasing use is being made of random coefficients structures, such as mixed logit, in the analysis of air travel choice behaviour. These models have the advantage of being able to retrieve random variations in sensitivities across travellers. An important issue, however, arises in the computation of willingness to pay indicators, such as the valuation of travel time savings, on the basis of randomly distributed coefficients. Indeed, with the standard approach of using simulation of the ratios across random draws, major problems can be caused by outliers, leading to biased trade-offs, which in turn lead to major issues in policy analyses. Here, a different approach is explored, making use of individual-specific draws from the random distributions, conditioned on the observed sequence of choices for each respondent. An analysis making use of stated preference data for airport and airline choice confirms the advantages of the approach using conditional draws, producing much more realistic distributional patterns for a range of willingness to pay indicators

Shear-stress controlled dynamics of nematic complex fluids

Based on a mesoscopic theory we investigate the non-equilibrium dynamics of a sheared nematic liquid, with the control parameter being the shear stress $\sigma_{\mathrm{xy}}$ (rather than the usual shear rate, $\dot\gamma$). To this end we supplement the equations of motion for the orientational order parameters by an equation for $\dot\gamma$, which then becomes time-dependent. Shearing the system from an isotropic state, the stress- controlled flow properties turn out to be essentially identical to those at fixed $\dot\gamma$. Pronounced differences when the equilibrium state is nematic. Here, shearing at controlled $\dot\gamma$ yields several non-equilibrium transitions between different dynamic states, including chaotic regimes. The corresponding stress-controlled system has only one transition from a regular periodic into a stationary (shear-aligned) state. The position of this transition in the $\sigma_{\mathrm{xy}}$-$\dot\gamma$ plane turns out to be tunable by the delay time entering our control scheme for $\sigma_{\mathrm{xy}}$. Moreover, a sudden change of the control method can {\it stabilize} the chaotic states appearing at fixed $\dot\gamma$.Comment: 10 pages, 11 figure

Treatment of reference alternatives in stated choice surveys for air travel choice behaviour

Stated Choice (SC) surveys are increasingly being used instead of Revealed Preference (RP) surveys for the study of air travel choice behaviour. In many cases, the choice situations presented in these SC surveys are constructed around an observed trip, where this is often included as one of the alternatives. Classically, these RP alternatives have been treated in the same way as the SC alternatives. The applications presented in this paper show that this potentially leads to biased results, and that it is important to recognise the differences in the nature of the two types of alternative. Additionally, the paper discusses issues caused by respondents who consistently prefer the RP alternative over the SC alternatives, a common phenomenon in such SC data

Modelling airport and airline choice behaviour with the use of stated preference survey data

The majority of studies of air travel choice behavior make use of revealed preference (RP) data, generally in the form of survey data collected from departing passengers. While the use of RP data has certain methodological advantages over the use of stated preference (SP) data, major issues arise because of the often low quality of the data relating to the un-chosen alternatives, in terms of explanatory variables as well as availability. As such, studies using RP survey data often fail to recover a meaningful fare coefficient, and are generally not able to offer a treatment of the effects of airline allegiance. In this paper, we make use of SP data for airport and airline choice collected in the US in 2001. The analysis retrieves significant effects relating to factors such as airfare, access time, flight time and airline and airport allegiance, illustrating the advantages of SP data in this context. Additionally, the analysis explores the use of non-linear transforms of the explanatory variables, as well as the treatment of continuous variations in choice behavior across respondents

Multiplicative structure in equivariant cohomology

We introduce the notion of a strongly homotopy-comultiplicative resolution of a module coalgebra over a chain Hopf algebra, which we apply to proving a comultiplicative enrichment of a well-known theorem of Moore concerning the homology of quotient spaces of group actions. The importance of our enriched version of Moore's theorem lies in its application to the construction of useful cochain algebra models for computing multiplicative structure in equivariant cohomology. In the special cases of homotopy orbits of circle actions on spaces and of group actions on simplicial sets, we obtain small, explicit cochain algebra models that we describe in detail.Comment: 28 pages. Final version (cosmetic changes, slight reorganization), to appear in JPA

Rational homotopy theory: a brief introduction

These notes contain a brief introduction to rational homotopy theory: its model category foundations, the Sullivan model and interactions with the theory of local commutative rings.Comment: A slight revision (some minor errors corrected) of lecture notes from a minicourse given in the summer school "Interactions between Homotopy Theory and Algebra," August 2004. (28 pages

Deduction over Mixed-Level Logic Representations for Text Passage Retrieval

A system is described that uses a mixed-level representation of (part of) meaning of natural language documents (based on standard Horn Clause Logic) and a variable-depth search strategy that distinguishes between the different levels of abstraction in the knowledge representation to locate specific passages in the documents. Mixed-level representations as well as variable-depth search strategies are applicable in fields outside that of NLP.Comment: 8 pages, Proceedings of the Eighth International Conference on Tools with Artificial Intelligence (TAI'96), Los Alamitos C