Simultaneous Equations and Weak Instruments under Conditionally Heteroscedastic Disturbances

In this paper we extend the setting analysed in Hahn and Hausman (2002a) by allowing for conditionally heteroscedastic disturbances. We start by considering the type of conditional variance-covariance matrices proposed by Engle and Kroner (1995) and we show that, when we impose a GARCH specification in the structural model, some conditions are needed to have a GARCH process of the same order in the reduced form equations. Later, we propose a modified-2SLS and a modified-3SLS procedures where the conditional heteroscedasticity is taken into account, that are more asymptotically efficient than the traditional 2SLS and 3SLS estimators. We recommend to use these modified-2SLS and 3SLS procedures in practice instead of alternative estimators like LIML/FIML, where the non-existence of moments leads to extreme values (in case we are interested in the structural form). We show theoretically and with simulation that in some occasions 2SLS, 3SLS and our proposed 2SLS and 3SLS procedures can have very severe biases, and we present the bias correction mechanisms to apply in practiceSimultaneous Equations, conditionally heteroscedastic disturbances

The estimation of simultaneous equation models under conditional heteroscedasticity

In this paper we extend the setting analysed in Hahn and Hausman (2002a) by allowing for conditionally heteroscedastic disturbances. We start by considering the type of conditional variance-covariance matrices proposed by Engle and Kroner (1995) and we show that, when we impose a GARCH specification in the structural model, some conditions are needed to have a GARCH process of the same order in the reduced form equations. Later, we propose a modified-2SLS and a modified-3SLS procedures where the conditional heteroscedasticity is taken into account, that are more asymptotically efficient than the traditional 2SLS and 3SLS estimators. We recommend to use these modified-2SLS and 3SLS procedures in practice instead of alternative estimators like LIML/FIML, where the non-existence of moments leads to extreme values (in case we are interested in the structural form). We show theoretically and with simulation that in some occasions 2SLS, 3SLS and our proposed 2SLS and 3SLS procedures can have very severe biases, and we present the bias correction mechanisms to apply in practiceSimultaneous equations, conditional heteroscedasticity

Increased GFAP immunoreactivity by astrocytes in response to contact with dorsal root ganglia cells in a 3D culture model

Failure of repair mechanisms in the injured CNS is widely attributed to the inhibitory environment of the lesion site, most notably the formation of the glial scar which forms a physical and physiological barrier to axon regeneration. We developed an in vitro 3D cell culture model to investigate the response of astrocytes to cells found at the inhibitory interfaces formed following damage to the spinal cord. CellTrackerTM labelled dissociated DRGs were seeded onto astrocyte-populated collagen gels and maintained in culture for 5 days. Astrocytes near the DRG interface showed marked GFAP up-regulation and adopted a reactive morphology which was observed up to 1mm away. Intensity of GFAP fluorescence at this interface was 3 fold higher than that seen away from the interface or in controls (astrocyte only gels). Furthermore, the presence of DRG conditioned medium was not capable alone of eliciting this response. In conclusion this model may provide a useful tool for understanding reactive astrogliosis in response to cells found at inhibitory interfaces following spinal cord or dorsal root injury. The contact between astrocytes and satellite cells may be enough to induce astrocyte reactivity and formation of the gliotic scar, or this contact may induce the secretion of a soluble factor which is not released from DRG cultures under physiological conditions

MULTIVARIATE ARCH MODELS: FINITE SAMPLE PROPERTIES OF ML ESTIMATORS AND AN APPLICATION TO AN LM-TYPE TEST

At the present time, there exists an important and growing econometric literature that deals with the application of multivariate-ARCH models to a variety of economic and financial data. However, the properties of the estimation procedures that are used have not yet been fully explored. This paper provides two main new results: the first concerns the large biases and variances that can arise when the ML estimation method is employed in a simple bivariate structure under the assumption of conditional heteroscedasticity; and the second examines how to use these analytical theoretical results to improve the size and the power of a test for multivariate ARCH effects. We analyse two models: one proposed in Wong and Li (1997) (where the disturbances are dependent but uncorrelated) and another proposed by Engle and Kroner (1995) and Liu and Polasek (1999, 2000) (where conditional correlation is allowed through a diagonal representation). We prove theoretically that a relatively large difference between the intercepts in the two conditional variance equations produces, in the first model, very large variances in some of the ML estimators and, in the second, very severe biases in some of the ML estimators of the parameters. Later we use our bias expressions to propose an LM type test of multivariate ARCH effects, showing that the size and the power of the test improve when we allow for bias correction in the estimators, and that the best recommendation in practical applications is always to use the expected hessian version of the LM. We address as well some constraints that should be included in the estimation of the models but which have so far been ignored. Finally, we present a SUR (seemingly unrelated) specification in both models, that provides an alternative way to retrieve the information matrix. We also extend Lumsdaine (1995) results in multivariate framework.Multivariate GARCH, Bias evaluation.

A Study of the Impact of Changing Business Processes and the Implementation of Computerised Systems on a Small Business’ Customer Relationships – a 2 Year Case Study on a Micro-organisation

This paper examines the impact of rolling out computerized systems within a micro organization focusing specifically on the impact of the roll out on customer relationships. The findings are based on a 2 year study on one micro-organization based in the North East of England, examining the nature of the implementation and the additional factors that affected the outcome and eventual detrimental impact on the relationship between the organization and the customer based. Alternative strategies will be discussed and redemption options examined. The primary research findings are based on observational evidence and interviews of the customer base, the researcher worked within the organization and were party to the business logic applied to the decisions made during the deployment perio

Empirical Validation of an Executive Function Battery for use in Childhood and Adolescence

This research explores the reliability and efficacy of a child and adolescent adaptation of an adult battery of executive functioning, measuring the constructs of reasoning, short-term memory and verbal processing. The intent of the research is twofold as it intends to support an age appropriate adjustment of a battery of tasks presented by Hampshire, Highfield, Parkin and Owen (2012), and secondly to display the necessity of looking at executive functions as multifaceted and therefore requiring multiple tasks to encompass their complexities. The adjusted battery in analysis is composed of nine tasks that have been amended to be age appropriate for a sample of children aged 8, and adolescents (15-17). These tasks were administered to a group of 8- year- olds recruited through the London Child Development Pool with task completion facilitated online, as well as to a large group of adolescents participating in a study on healthy relationships conducted by the Center of Addiction and Mental Health in London, Ontario. A descriptive comparison of means was conducted to establish that an accurate developmental trajectory of the constructs of reasoning, short-term memory and verbal processing exists with the use of the age adjusted battery. A correlational analysis was completed with the Multidimensional Aptitude Battery- II in the adolescent cohort, which supported the reliable transition of the constructs of reasoning and short-term memory but raised concerns with the accuracy of the verbal processing component. Limitations of the study were discussed as well as future directions for this research

Operads, Symmetric Monoidal Categories, and Localizations

An operad can be thought of as a collection of operations, each with a finite number of inputs and a single output, along with a composition rule. We prove that the category of operads in an appropriate concrete symmetric monoidal category V is equivalent to a subcategory of symmetric monoidal categories enriched in V. Though versions of this result have appeared previously in the literature, we prove that a more restrictive subcategory is needed to construct the equivalence. Our subcategory has the advantage that its objects share important properties with the historical precursor to operads, PROPs. We also review a localization construction for operads, called the tree hammock localization. Using the above equivalence, we compare this construction to the hammock localization for categories. We believe that these two localization constructions should be suitably equivalent, and present ongoing work on this conjecture using simplicial categories and ∞-categories

The effects of a UK review of breast cancer screening on uptake : an observational before/after study

Objectives: To measure whether uptake of breast cancer screening was affected by the publication of the Marmot Review and associated press coverage. Setting: Eight NHS breast screening centres in the West Midlands of the UK. Methods: Uptake of breast cancer screening invitations was compared in the week before and after the Marmot review publication. All 12,023 women invited for screening between 23 October 2012 and 5 November 2012 were included. A mixed effects model of the predictors of screening uptake (on date invited, or within 21 days) was created. Predictors considered for inclusion were whether the allocated screening appointment was before or after publication of the review, population factors (age, index of multiple deprivation income domain by quintile, previous attendance), and interaction terms. Results: Uptake decreased after publication of the review from 65% to 62% (OR = 0.87 95%CI = 0.80–0.94), but a similar decrease was seen for the same dates on the previous year (OR = 0.85 95%CI = 0.78–0.93). Odds of attending screening were lower for women in the most deprived (uptake = 49%, OR = 0.54, 95%CI = 0.46–0.62) in comparison with the least deprived quintile (uptake = 71%). Odds of attendance also increased if the woman had ever previously attended (OR 3.9 95% CI 3.5–4.4), and decreased with each year of increasing age (OR 0.96 95% CI 0.96–0.97). There were no interactions between any of the other predictors and whether the appointment was before or after publication of the Marmot review. Conclusion: No change in uptake of breast cancer screening above normal seasonal variation was detected after publication of the Marmot review