Simplicial minisuperspace models in the presence of a massive scalar field with arbitrary scalar coupling

We extend previous simplicial minisuperspace models to account for arbitrary scalar coupling \eta R\phi^2.Comment: 24 pages and 9 figures. Accepted for publication by Classical and Quantum Gravit

On the Nonrelativistic Limit of the phi^4 Theory in 2+1 Dimensions

We study the nonrelativistic limit of the quantum theory of a real scalar field with quartic self-interaction. The two body scattering amplitude is written in such way as to separate the contributions of high and low energy intermediary states. From this result and the two loop computation of the self energy correction, we determine an effective nonrelativistic action.Comment: 15 pages, 2 figures, revte

Anisotropic simplicial minisuperspace model

The computation of the simplicial minisuperspace wavefunction in the case of anisotropic universes with a scalar matter field predicts the existence of a large classical Lorentzian universe like our own at late timesComment: 19 pages, Latex, 6 figure

Identification issues in models for underreported counts

In this note we study the conditions under which leading models for underreported counts are identified. In particular, we highlight a peculiar identification problem that afflicts two of the most popular models in this class.

Currency Unions in Prospect and Retrospect

We critically review the recent literature on currency unions, and discuss the methodological challenges posed by the empirical assessment of their costs and benefits. In the process, we provide evidence on the economic effects of the euro. In particular, and in contrast with estimates of the trade effect of other currency unions, we find that the impact of the euro on trade has been close to zero. After reviewing the costs and benefits, we conclude with some open questions on normative and positive aspects of the theory of currency unions, emphasizing the need for a unified welfare-based framework to weigh their costs and gains.Currency union, Integration, Exchange Rage, Trade

Quantiles for Counts

This paper studies the estimation of conditional quantiles of counts. Given the discreteness of the data, some smoothness has to be artificially imposed on the problem. The methods currently available to estimate quantiles of count data either assume that the counts result from the discretization of a continuous process, or are based on a smoothed objective function. However, these methods have several drawbacks. We show that it is possible to smooth the data in a way that allows inference to be performed using standard quantile regression techniques. The performance and implementation of the estimator are illustrated by simulations and an application.Asymmetric maximum likelihood, Jittering, Maximum score estimator, Quantile regression, Smoothing.

Trading Partners and Trading Volumes: Implementing the Helpman-Melitz-Rubinstein Model Empirically

Helpman, Melitz, and Rubinstein (2008)-HMR-present a rich theoretical model to study the determinants of bilateral trade flows across countries. The model is then empirically implemented through a two-stage estimation procedure. This note seeks to clarify some econometric aspects of the estimation approach used by HMR and explore the consequences of possible departures from the maintained distributional assumptions.Gravity equation, Heteroskedasticity, Jensens inequality

On the Existence of the Maximum Likelihood Estimates for Poisson Regression

We note that the existence of the maximum likelihood estimates for Poisson regression depends on the data configuration. Because standard software does not check for this problem, the practitioner may be surprised to find that in some applications estimation of the Poisson regression is unusually difficult or even impossible. More seriously, the estimation algorithm may lead to spurious maximum likelihood estimates. We identify the signs of the non-existence of the maximum likelihood estimates and propose a simple empirical strategy to single out the regressors causing this type of identification failure.Poisson estimation, gravity equation