6,090 research outputs found
The wild bootstrap for multilevel models
In this paper we study the performance of the most popular bootstrap schemes
for multilevel data. Also, we propose a modified version of the wild bootstrap
procedure for hierarchical data structures. The wild bootstrap does not require
homoscedasticity or assumptions on the distribution of the error processes.
Hence, it is a valuable tool for robust inference in a multilevel framework. We
assess the finite size performances of the schemes through a Monte Carlo study.
The results show that for big sample sizes it always pays off to adopt an
agnostic approach as the wild bootstrap outperforms other techniques
When orthography is not enough: the effect of lexical stress in lexical decision.
Three lexical decision experiments were carried out in Italian, in order to verify if stress dominance (the most frequent stress type) and consistency (the proportion and number of existent words sharing orthographic ending and stress pattern) had an effect on polysyllabic word recognition. Two factors were manipulated: whether the target word carried stress on the penultimate (dominant; graNIta, seNIle 'slush, senile') or on the antepenultimate (non-dominant) syllable (MISsile, BIbita 'missile, drink'), and whether the stress neighborhood was consistent (graNIta, MISsile) or inconsistent (seNIle, BIbita) with the word\u2019s stress pattern. In Experiment 1 words were mixed with nonwords sharing the word endings, which made words and nonwords more similar to each other. In Experiment 2 words and nonwords were presented in lists blocked for stress pattern. In Experiment 3 we used a new set of nonwords, which included endings with (stress) ambiguous neighborhoods and/or with low number of neighbors, and which were overall less similar to words. In all three experiments there was an advantage for words with penultimate (dominant) stress, and no main effect of stress neighborhood. However, the dominant stress advantage decreased in Experiments 2 and 3. Finally, in Experiment 4 the same materials used in Experiment 1 were also used in a reading aloud task, showing a significant consistency effect, but no dominant stress advantage. The influence of stress information in Italian word recognition is discussed
Multilevel Models with Stochastic Volatility for Repeated Cross-Sections: an Application to tribal Art Prices
In this paper we introduce a multilevel specification with stochastic
volatility for repeated cross-sectional data. Modelling the time dynamics in
repeated cross sections requires a suitable adaptation of the multilevel
framework where the individuals/items are modelled at the first level whereas
the time component appears at the second level. We perform maximum likelihood
estimation by means of a nonlinear state space approach combined with
Gauss-Legendre quadrature methods to approximate the likelihood function. We
apply the model to the first database of tribal art items sold in the most
important auction houses worldwide. The model allows to account properly for
the heteroscedastic and autocorrelated volatility observed and has superior
forecasting performance. Also, it provides valuable information on market
trends and on predictability of prices that can be used by art markets
stakeholders
What Regulates Galaxy Evolution? Open Questions in Our Understanding of Galaxy Formation and Evolution
In April 2013, a workshop entitled "What Regulates Galaxy Evolution" was held
at the Lorentz Center. The aim of the workshop was to bring together the
observational and theoretical community working on galaxy evolution, and to
discuss in depth of the current problems in the subject, as well as to review
the most recent observational constraints. A total of 42 astrophysicists
attended the workshop. A significant fraction of the time was devoted to
identifying the most interesting "open questions" in the field, and to discuss
how progress can be made. This review discusses the four questions (one for
each day of the workshop) that, in our opinion, were the focus of the most
intense debate. We present each question in its context, and close with a
discussion of what future directions should be pursued in order to make
progress on these problems.Comment: 36 pages, 6 Figures, submitted to New Astronomy Review
On the scatter in the relation between stellar mass and halo mass: random or halo formation time dependent?
The empirical HOD model of Wang et al. 2006 fits, by construction, both the
stellar mass function and correlation function of galaxies in the local
Universe. In contrast, the semi-analytical models of De Lucia & Blazoit 2007
(DLB07) and Guo et al. 2011 (Guo11), built on the same dark matter halo merger
trees than the empirical model, still have difficulties in reproducing these
observational data simultaneously. We compare the relations between the stellar
mass of galaxies and their host halo mass in the three models, and find that
they are different. When the relations are rescaled to have the same median
values and the same scatter as in Wang et al., the rescaled DLB07 model can fit
both the measured galaxy stellar mass function and the correlation function
measured in different galaxy stellar mass bins. In contrast, the rescaled Guo11
model still over-predicts the clustering of low-mass galaxies. This indicates
that the detail of how galaxies populate the scatter in the stellar mass --
halo mass relation does play an important role in determining the correlation
functions of galaxies. While the stellar mass of galaxies in the Wang et al.
model depends only on halo mass and is randomly distributed within the scatter,
galaxy stellar mass depends also on the halo formation time in semi-analytical
models. At fixed value of infall mass, galaxies that lie above the median
stellar mass -- halo mass relation reside in haloes that formed earlier, while
galaxies that lie below the median relation reside in haloes that formed later.
This effect is much stronger in Guo11 than in DLB07, which explains the
over-clustering of low mass galaxies in Guo11. Our results illustrate that the
assumption of random scatter in the relation between stellar and halo mass as
employed by current HOD and abundance matching models may be problematic in
case a significant assembly bias exists in the real Universe.Comment: 10 pages, 6 figures, published in MNRA
A first thermodynamic interpretation of the technology transfer activities
In the last years new interdisciplinary approaches to economics and social
science have been developed. A Thermodynamic approach to socio-economics has
brought to a new interdisciplinary scientific field called econophysics. Why
thermodynamic? Thermodynamic is a statistical theory for large atomic system
under constraints of energy[1] and the economy can be considered a large system
governed by complex rules. The present job proposes a new application, starting
from econophysic, passing throughout the thermodynamic laws to interpret and to
described the Technology Transfer (TT) activities. Using the definition of
economy (i.e. economy[dictionary def.] = the process or system by which goods
and services are produced, sold, and bought in a country or region) the TT can
be considered an important sub-domain of the economy and a transversal new area
of the scientific research. The TT is the process of transferring knowledge,
that uses the results from the research to produce innovation and to ensure
that scientific and technological developments could become accessible to a
wider range of users. Starting from important Universities (MIT, Stanford,
Oxford, etc) nowadays the TT is assuming a central role. It is called the third
mission, together with education and research. The importance to provide new
theories and tools to describe the TT activities and their behavior, has been
retained fundamental to support the social rapid evolution that is involving
the TT offices. The presented work uses the thermodynamic theories applying
them to Technology Transfer and starting from the concept of entropy, exergy
and anergy. The output analysis should become an help to make decision to
improve the TT activities and a better resources employment
Hierarchization of the Italian region on the strength of the agricultural mechanization through clustering analysis
The aim of this paper has been to study the organization of the Italian agricultural enterprises through a cluster analysis. Starting from statistical data, the Italian Regions were then classified into homogeneous groups in proportion with the size of the farms, their agricultural mechanization level and the manpower employment. The suitability of this arrangement was supported by the variability among the groups, which was greater than that within the groups. Generally each group is formed both by adjacent and non-adjacent Regions and also by Regions geographically distant. A concise but clear picture pertaining the different structure of Italian farms were was pointed out
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