Bayesian non-parametric inference for species variety with a two-parameter Poisson–Dirichlet process prior

Summary.  A Bayesian non-parametric methodology has been recently proposed to deal with the issue of prediction within species sampling problems. Such problems concern the evaluation, conditional on a sample of size n , of the species variety featured by an additional sample of size m . Genomic applications pose the additional challenge of having to deal with large values of both n and m . In such a case the computation of the Bayesian non-parametric estimators is cumbersome and prevents their implementation. We focus on the two-parameter Poisson–Dirichlet model and provide completely explicit expressions for the corresponding estimators, which can be easily evaluated for any sizes of n and m . We also study the asymptotic behaviour of the number of new species conditionally on the observed sample: such an asymptotic result, combined with a suitable simulation scheme, allows us to derive asymptotic highest posterior density intervals for the estimates of interest. Finally, we illustrate the implementation of the proposed methodology by the analysis of five expressed sequence tags data sets.     

The Size and Power of Bootstrap and Bartlett-Corrected Tests of Hypotheses on the Cointegrating Vectors

In this paper we compare Bartlett-corrected, bootstrap, and fast double bootstrap tests on maximum likelihood estimates of cointegration parameters. The key result is that both the bootstrap and the Bartlett-corrected tests must be based on the unrestricted estimates of the cointegrating vectors: procedures based on the restricted estimates have almost no power. The small sample size bias of the asymptotic test appears so severe as to advise strongly against its use with the sample sizes commonly available; the fast double bootstrap test minimizes size bias, while the Bartlett-corrected test is somehow more powerful.     

Lot sizing with storage losses under demand uncertainty

We address a variant of the single item lot sizing problem affected by proportional storage (or inventory) losses and uncertainty in the product demand. The problem has applications in, among others, the energy sector, where storage losses (or storage deteriorations) are often unavoidable and, due to the need for planning ahead, the demands can be largely uncertain. We first propose a two-stage robust optimization approach with second-stage storage variables, showing how the arising robust problem can be solved as an instance of the deterministic one. We then consider a two-stage approach where not only the storage but also the production variables are determined in the second stage. After showing that, in the general case, solutions to this problem can suffer from acausality (or anticipativity), we introduce a flexible affine rule approach which, albeit restricting the solution set, allows for causal production plans. A hybrid robust-stochastic approach where the objective function is optimized in expectation, as opposed to in the worst-case, while retaining robust optimization guarantees of feasibility in the worst-case, is also discussed. We conclude with an application to heat production, in the context of which we compare the different approaches via computational experiments on real-world data.     

Twenty-Five Years of Materialism: Do the US and Europe Diverge?

Using data from the World Values Survey and the European Values Study, we compare the trends of materialism over the last quarter of century among the US and six major European countries: France, Spain, Italy, Germany, Great Britain and Sweden. We use the definition of materialism adopted by positive psychologists. We find that the trends in Europe and in the US diverged. In the US materialism increased, while in Europe it decreased. However, some mixed patterns arise. In particular, Great Britain, Spain and Sweden showed some symptoms of an increase of materialistic values, although they were far less pronounced compared to the American ones. As far as the levels of materialism are concerned, it is interesting that, according to most of our measures, Americans were relatively less materialistic at the beginning of our period of observation. Yet, towards the end of the period they scored very high in the ranking of materialism in our sample of countries.     

A mortality model based on a mixture distribution function

A new mortality model based on a mixture distribution function is proposed. We mix a half-normal distribution with a generalization of the skew-normal distribution. As a result, we get a six-parameter distribution function that has a good fit with a wide variety of mortality patterns. This mixture model is fitted to several mortality data schedules and compared with the Siler (five-parameter) and Heligman–Pollard (eight-parameter) models. Our proposal serves as a convenient compromise between the Heligman–Pollard model (which ensures a good fit with data but is often overparameterized) and the Siler model (which is more compact but fails to capture ‘accident humps’).     

Fragile scenes,fractured communities: Tunisian Metal and sceneness

The paper proposes a reshaping of musical and cultural scene as a framework for the study of youth cultures. Developed in the ambit of post-subcultural theories, scene well represents the forces and flaws of such a category: high dynamism and ethnographic richness on one side; vagueness on the other. In order to reduce such vagueness, I conceptualise sceneness, intended as the substance of scenes, their density in networking and infrastructures. I use Arjun Appadurai’s concepts of locality and neighbourhoods to signify, respectively, sceneness and the actual scenes: in this way I redefine scene as a fragile construct that needs to be ritually revived, and that can work as a context for the development of new meanings and social groups. This implies conflict as a central element of scenes, one which can lead them to disequilibrium and disappearance. Such a redefinition is helpful for analysing fragile scenes which struggle to exist in troubled contexts, such as poor and hostile social environments. I provide, as an example, my ethnographic research on Metal in Tunisia. Caught between idealised images of community and an actual community which was conflictual and ‘fractured’, the Tunisian Metal scene lives a precarious existence threatened by material constraints and cultural marginality.     

Initial agronomic performances of Mediterranean xerophytes in simulated dry green roofs

The growing desire to make the urban environment more sustainable from an ecological point of view has stimulated research on the architectural and agronomic aspects of green roofs. The practical realisation of green roofs, is however limited by economic and ecological issues. More specifically, water availability is the most limiting factor, and is likely to be ever more so in the future in the light of climate change. For this reason, we evaluated the agronomic performance of several xerophytes in a simulated dry green roof. Seeds of 20 species were collected in typically dry habitats (abandoned quarries, rocky soils, dunes, etc.) and studied in the laboratory for germination ecology. In cases of strong dormancy, methods were tested to stimulate germination and their germination ecology was studied. The resulting seedlings were transplanted in spring 2008 in two green roof types that differ in substrate depth (150 and 200 mm) made up of lapil, pumice, zeolites and peat, resting on a drainage layer of hydroperlite. Temperature and humidity in the substrate and drainage layer were measured during the whole test period. Survival of the seedlings in both substrate depths was almost 100%, favoured by a rainy spring. Most of the tested species showed an excellent performance during the hot and dry summer months in terms of survival rates, growth, and vegetation cover dynamics, notwithstanding the difficult ecological conditions (temperatures around 50°C; hydric potential Ψ -15 bars). Furthermore, most of the species had a long flowering stage in the first year of growth. Plants in the green roof with the deeper substrate depth produced, for most of the tested taxa, a significantly higher vegetation cover and growth compared to when they were placed in the 150 mm substrate. The results of this study show that some Mediterranean xerophytes have biological characteristics suitable for their use in dry green roofs, although an irrigation system for emergency use seems advisable. To conclude, further research should focus on long term evaluation of green roof vegetation in terms of plant survival and flowering dynamics.     

A mind in a disk: the attribution of mental states to technological systems

This paper reports a study about the role of different variables in the process of attributing mental states to technological systems, variables such as the number of figural elements displayed in the system and the personality traits of the subjects interacting with the systems. In an experiment, participants were interacting with a computer on whose screen several disks of various sizes and colours were blinking at different rates. Each time a disk reappeared on the screen its position was randomly varied. As in a videogame, participants had to click on the disks to increase their score. The results showed that, even in the case of such a simple system, subjects believed that the figural elements they were interacting with had some form of mental states, although their confidence in these beliefs varied in the different experimental conditions. The confidence level of the attributions, in fact, was not the same for all the different mental states considered, and it varied also both with the number of elements being displayed as well as with some personality traits of the subjects.     

DEBT,POLICY UNCERTAINTY,AND EXPECTATIONS STABILIZATION

This paper develops a model of policy regime uncertainty and its consequences for stabilizing expectations. Because of learning dynamics, uncertainty about monetary and fiscal policy is shown to restrict, relative to a rational expectations analysis, the set of policies consistent with macroeconomic stability. Anchoring expectations by communicating about monetary and fiscal policy enlarges the set of policies consistent with stability. However, absent anchored fiscal expectations, the advantages from anchoring monetary expectations are smaller the larger is the average level of indebtedness. Finally, even when expectations are stabilized in the long run, the higher are average debt levels the more persistent will be the effects of disturbances out of rational expectations equilibrium.     

BENIGN NEGLECT OF COVENANT VIOLATIONS: BLISSFUL BANKING OR IGNORANT MONITORING?

Theoretically, bank's loan monitoring activity hinges critically on its capitalization. To proxy for monitoring intensity, we use changes in borrowers' investment following loan covenant violations, when creditors can intervene in the governance of the firm. Exploiting granular bank‐firm relationships observed in the syndicated loan market, we document substantial heterogeneity in monitoring across banks and through time. Better capitalized banks are more lenient monitors that intervene less with covenant violators. Importantly, this hands‐off approach is associated with improved borrowers' performance. Beyond enhancing financial resilience, regulation that requires banks to hold more capital may thus also mitigate the tightening of credit terms when firms experience shocks. (JEL G21, G32, G33, G34)