Testing the exogeneity assumption in panel data models with "non classical" disturbances

This paper is concerned with the use of the Durbin-Wu-Hausman test for correlated effects with panel data. The assumptions underlying the construction of the statistic are too strong in many empirical cases. The consequences of deviations from the basic assumptions are investigated. The size distortion is assessed. In the case of measurement error, the Hausman test is found to be a test of the difference in asymptotic biases of between and within group estimators. However, its `size' is sensitive to the relative magnitude of the intra-group and inter-group variations of the covariates, and can be so large as to preclude the use of the statistic in this case. We show to what extent some assumptions can be relaxed in a panel data context and we discuss an alternative robust formulation of the test. Power considerations are presented. Keywords; models with panel data, hausman test, minimum variance estimators, quadratic forms in normal variables, monte carlo simulations

The Size and Power of Bootstrap Tests for Linear Restrictions in Misspecified Cointegrating Relationships

This paper considers computer intensive methods for inference on cointegrating vectors in maximum likelihood analysis. It investigates the robustness of LR , Wald tests and an F-type test for linear restrictions on cointegrating space to misspecification on the number of cointegrating relations. In addition, since all the distributional results within the maximum likelihood cointegration model rely on asymptotic considerations, it is important to consider the sensitivity of inference procedures to the sample size. In this paper we use bootstrap hypothesis testing as a way to improve inference for linear restriction on the cointegrating space. We find that the resampling procedure is a very useful device for tests that lack the invariance property such as the Wald test, where the size distortion of the bootstrap test converges to zero even for a sample size T=50. Moreover, it turns out that when the number of cointegrating vectors are correctly specified the bootstrap succeeds where the asymptotic approximation is not satisfactory, that is, for a sample size T

The centralised development of elearning resources : a thesis presented in partial fulfilment of the requirements for the Masters of Education (Elearning) at Massey University, Manawatū, New Zealand

This thesis explores the centralisation of elearning resource development in New Zealand Institutes of Technology and Polytechnics (ITPs). There was a significant gap in existing research relating to the organisation of elearning resource development. The tertiary education sector has been subject to significant and rapid change with associated challenges. Centralisation has been mooted as contributing to a solution for these challenges. The lack of research around centralised development of resources makes it difficult to support such a claim. To address this, the thesis explored three areas: the extent to which centralised development of elearning has been adopted, the perceived advantages and disadvantages of a centralised model, and the attitudes teaching staff hold towards a centralised model. The study applied a mixed method convergent parallel research design. This drew on data from interviews with elearning managers and from a survey of teaching staff. Findings established that three categories of centralisation exist in New Zealand ITPs; decentralised, centralised and highly centralised. The typical composition and functions of the centralised teams were defined for each category. The findings supported the perceived advantages and disadvantages identified in existing research, but also identified additional advantages. These included better project management, more clarity around roles and responsibilities, that elearning resources produced by a centralised unit was more student focussed and specific cost saving information. Levels of understanding around the financial advantages of a centralised model were inconsistent. The attitudes teaching staff held towards a centralised model were seen as to some extent ambivalent. Attitudes were more positive where the staff already operated within a centralised model. The thesis makes a significant contribution where there was a gap in existing research. This new knowledge is directly relevant to current decisions around cost of development, composition of central teams, expectations when adopting a centralised model, and planning to centralise or decentralise. These findings are both timely and significant as recent mergers, qualification reviews and the expectation to innovate and adopt new models of delivery increase the need for more efficient solutions to creating elearning resources

Building the Terrestrial Planets: Constrained Accretion in the Inner Solar System

To date, no accretion model has succeeded in reproducing all observed constraints in the inner Solar System. These constraints include 1) the orbits, in particular the small eccentricities, and 2) the masses of the terrestrial planets -- Mars' relatively small mass in particular has not been adequately reproduced in previous simulations; 3) the formation timescales of Earth and Mars, as interpreted from Hf/W isotopes; 4) the bulk structure of the asteroid belt, in particular the lack of an imprint of planetary embryo-sized objects; and 5) Earth's relatively large water content, assuming that it was delivered in the form of water-rich primitive asteroidal material. Here we present results of 40 high-resolution (N=1000-2000) dynamical simulations of late-stage planetary accretion with the goal of reproducing these constraints, although neglecting the planet Mercury. We assume that Jupiter and Saturn are fully-formed at the start of each simulation, and test orbital configurations that are both consistent with and contrary to the "Nice model." We find that a configuration with Jupiter and Saturn on circular orbits forms low-eccentricity terrestrial planets and a water-rich Earth on the correct timescale, but Mars' mass is too large by a factor of 5-10 and embryos are often stranded in the asteroid belt. A configuration with Jupiter and Saturn in their current locations but with slightly higher initial eccentricities (e = 0.07-0.1) produces a small Mars, an embryo-free asteroid belt, and a reasonable Earth analog but rarely allows water delivery to Earth. None of the configurations we tested reproduced all the observed constraints. (abridged)Comment: Accepted to Icarus. 21 pages, 12 figures, 5 tables in emulateapj format. Figures 3 and 4 degraded. For full-resolution see http://casa.colorado.edu/~raymonsn/ms_emulateapj.pd

Water Delivery and Giant Impacts in the 'Grand Tack' Scenario

A new model for terrestrial planet formation (Hansen 2009, Walsh et al. 2011) has explored accretion in a truncated protoplanetary disk, and found that such a configuration is able to reproduce the distribution of mass among the planets in the Solar System, especially the Earth/Mars mass ratio, which earlier simulations have generally not been able to match. Walsh et al. tested a possible mechanism to truncate the disk--a two-stage, inward-then-outward migration of Jupiter and Saturn, as found in numerous hydrodynamical simulations of giant planet formation. In addition to truncating the disk and producing a more realistic Earth/Mars mass ratio, the migration of the giant planets also populates the asteroid belt with two distinct populations of bodies--the inner belt is filled by bodies originating inside of 3 AU, and the outer belt is filled with bodies originating from between and beyond the giant planets (which are hereafter referred to as `primitive' bodies). We find here that the planets will accrete on order 1-2% of their total mass from primitive planetesimals scattered onto planet-crossing orbits during the formation of the planets. For an assumed value of 10% for the water mass fraction of the primitive planetesimals, this model delivers a total amount of water comparable to that estimated to be on the Earth today. While the radial distribution of the planetary masses and the dynamical excitation of their orbits are a good match to the observed system, we find that the last giant impact is typically earlier than 20 Myr, and a substantial amount of mass is accreted after that event. However, 5 of the 27 planets larger than half an Earth mass formed in all simulations do experience large late impacts and subsequent accretion consistent with the dating of the Moon-forming impact and the estimated amount of mass accreted by Earth following that event

A measure of core inflation in the UK

We develop a measure of core inflation in the UK over the period January1987 - December 1998, following the work of Bryan and Cecchetti (1996),Bryan, Cecchetti and Wiggins (1997) and Roger (1997). Disaggregation is into85 price categories. Given the high kurtosis of the price change distribution over this period, a trimmed mean is a more robust estimator of core inflation than other published measures such as the RPIX widely used in the UK, in particular by the Bank of England when targeting inflation. We discuss the relative advantages of our measure of core inflation, with emphasis on the determination of an optimal trimming point and on the analysis of the products whose price changes are excluded from our measure in each time period. The resulting measure appears well behaved, but differs noticeably from a number of published measures

Small crater populations on Vesta

The NASA Dawn mission has extensively examined the surface of asteroid Vesta, the second most massive body in the main belt. The high quality of the gathered data provides us with an unique opportunity to determine the surface and internal properties of one of the most important and intriguing main belt asteroids (MBAs). In this paper, we focus on the size frequency distributions (SFDs) of sub-kilometer impact craters observed at high spatial resolution on several selected young terrains on Vesta. These small crater populations offer an excellent opportunity to determine the nature of their asteroidal precursors (namely MBAs) at sizes that are not directly observable from ground-based telescopes (i.e., below ~100 m diameter). Moreover, unlike many other MBA surfaces observed by spacecraft thus far, the young terrains examined had crater spatial densities that were far from empirical saturation. Overall, we find that the cumulative power-law index (slope) of small crater SFDs on Vesta is fairly consistent with predictions derived from current collisional and dynamical models down to a projectile size of ~10 m diameter (Bottke et al., 2005a,b). The shape of the impactor SFD for small projectile sizes does not appear to have changed over the last several billions of years, and an argument can be made that the absolute number of small MBAs has remained roughly constant (within a factor of 2) over the same time period. The apparent steady state nature of the main belt population potentially provides us with a set of intriguing constraints that can be used to glean insights into the physical evolution of individual MBAs as well as the main belt as an ensemble.Comment: Accepted by PSS, to appear on Vesta cratering special issu

A low mass for Mars from Jupiter's early gas-driven migration

Jupiter and Saturn formed in a few million years (Haisch et al. 2001) from a gas-dominated protoplanetary disk, and were susceptible to gas-driven migration of their orbits on timescales of only ~100,000 years (Armitage 2007). Hydrodynamic simulations show that these giant planets can undergo a two-stage, inward-then-outward, migration (Masset & Snellgrove 2001, Morbidelli & Crida 2007, Pierens & Nelson 2008). The terrestrial planets finished accreting much later (Klein et al. 2009), and their characteristics, including Mars' small mass, are best reproduced by starting from a planetesimal disk with an outer edge at about one astronomical unit from the Sun (Wetherill 1978, Hansen 2009) (1 AU is the Earth-Sun distance). Here we report simulations of the early Solar System that show how the inward migration of Jupiter to 1.5 AU, and its subsequent outward migration, lead to a planetesimal disk truncated at 1 AU; the terrestrial planets then form from this disk over the next 30-50 million years, with an Earth/Mars mass ratio consistent with observations. Scattering by Jupiter initially empties but then repopulates the asteroid belt, with inner-belt bodies originating between 1 and 3 AU and outer-belt bodies originating between and beyond the giant planets. This explains the significant compositional differences across the asteroid belt. The key aspect missing from previous models of terrestrial planet formation is the substantial radial migration of the giant planets, which suggests that their behaviour is more similar to that inferred for extrasolar planets than previously thought.Comment: 12 pages, 4 figures + Supplementary Material 46 pages, 10 figure