Perturbation selection and influence measures in local influence analysis

Cook's [J. Roy. Statist. Soc. Ser. B 48 (1986) 133--169] local influence approach based on normal curvature is an important diagnostic tool for assessing local influence of minor perturbations to a statistical model. However, no rigorous approach has been developed to address two fundamental issues: the selection of an appropriate perturbation and the development of influence measures for objective functions at a point with a nonzero first derivative. The aim of this paper is to develop a differential--geometrical framework of a perturbation model (called the perturbation manifold) and utilize associated metric tensor and affine curvatures to resolve these issues. We will show that the metric tensor of the perturbation manifold provides important information about selecting an appropriate perturbation of a model. Moreover, we will introduce new influence measures that are applicable to objective functions at any point. Examples including linear regression models and linear mixed models are examined to demonstrate the effectiveness of using new influence measures for the identification of influential observations.Comment: Published in at http://dx.doi.org/10.1214/009053607000000343 the Annals of Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical Statistics (http://www.imstat.org

Endogenous Leadership: Selection and Influence

In social dilemmas, leading a team by making heroic efforts may prove costly, especially if the followers are not adequately motivated to make similar sacrifices. Attempting to understand what motivates these seemingly selfless individuals to lead, we report the results of a two-stage public good experiment with endogenous timing. Even though it turns out to be costly on average, a large proportion of our subjects volunteer to lead. Our findings suggest that a fraction of these leaders are socially concerned, while others expect to distill some personal gain, possibly of non-pecuniary nature. The composition of the team also matters, as publicizing certain attributes of a subject's teammates has an impact on her decision to lead. Lastly, though voluntary leaders improve efficiency in their team, they are not necessarily more influential than randomly imposed leaders.leadership; endogenous selection; influence; voluntary contribution; experiment

Endogenous Leadership Selection and Influence

When it examines the risk of coordinated effects, an antitrust authority will usually compare the situation where the merger is accepted with an attendant risk of collusion with the benchmark case in which competition is present ex-post. The main objective of this paper is to show that the antitrust authority must take into account the possibility for firms to collude if a merger is rejected. In fact, firms can have incitations to make collusion ex-post (after a rejection of a merger) whereas they would not make collusion ex-ante. All the papers on mergers and collusion tend to look at a minimal discount factor threshold for collusion to be sustained. This article does not only suggest necessary and sufficient conditions for collusion to be enforced but it also analyses the choice which firms have as to whether to collude. We consider an industry with cost-asymmetric firms and we study the analysis of collusion under leniency programmes.endogenous switching models ; experiment ; influence ; leadership ; voluntary contribution

The influence of technology, environment and user acceptance on the effectiveness of information system project selection using SEM

The selection of the present information system project is difficult because of the many factors that influence it. Information system project should pay attention to the user acceptance, technology and the environment in terms of their influence on the information system project selection.The purpose of this paper is to determine how much influence user acceptance, technology and the environment have on the information system project selection.This research uses data obtained from several ministries and analyzed using SEM (Structural Equation Models).The results found that the technology and the environment affects user acceptance. Moreover, technology and environment affect the effectiveness of the information systems project selection through the mediating effect of user acceptance. User acceptance, tested by the incorporation of usefulness and ease of use, the results are more modest and in line with previous theories. Furthermore, the external environment highly impacts the information system project selection

Towards human control of robot swarms

In this paper we investigate principles of swarm control that enable a human operator to exert influence on and control large swarms of robots. We present two principles, coined selection and beacon control, that differ with respect to their temporal and spatial persistence. The former requires active selection of groups of robots while the latter exerts a passive influence on nearby robots. Both principles are implemented in a testbed in which operators exert influence on a robot swarm by switching between a set of behaviors ranging from trivial behaviors up to distributed autonomous algorithms. Performance is tested in a series of complex foraging tasks in environments with different obstacles ranging from open to cluttered and structured. The robotic swarm has only local communication and sensing capabilities with the number of robots ranging from 50 to 200. Experiments with human operators utilizing either selection or beacon control are compared with each other and to a simple autonomous swarm with regard to performance, adaptation to complex environments, and scalability to larger swarms. Our results show superior performance of autonomous swarms in open environments, of selection control in complex environments, and indicate a potential for scaling beacon control to larger swarms

Exploring performance attribution: the case of quality managment systems adoptions and business performance

Purpose. Accreditation to the ISO 9001 Quality Management Systems Standard has proven to be a persistent and growing phenomenon in services and manufacturing, yet to date little attempt has been made to explore how performance results in cross-sectional research may be attributed to different causation mechanisms and how their influences may alter over time. Methodology. The paper defines four possible causation mechanisms before searching and analysing the empirical literature on quality management system certification to ISO 9001 and business performance for evidence of their causal influence. Findings. From the analyses it is found that the benefit that can safely be attributed to the treatment-effect of ISO 9001 accreditation is lower waste; while the benefits of lower costs and better quality are less likely unless motives for adoption are developmental rather than externally driven. From an analysis of longitudinal studies a strong selection-mechanism is found where more profitable firms have a greater propensity to adopt than less profitable firms. From the finding propositions are developed to show how the influence of these mechanisms change over time. Implications for research. The existence of the selection-mechanism has profound implications for interpreting business performance achievements because the benefits that are attributed to the treatment-effect from adopting quality management system standards are likely to be greatly inflated by the influence of the selection-mechanism. The author suggests that richer theory is needed that can incorporate bi-directional influences and new research is needed to explore the underlying causes of the selection effect. Value of paper. The paper is believed to be the first to systematically explore attribution of performance in the ISO 9001 literature. Its findings provide new insights into the complexities of attribution of performance in studies of new practices and systems. Keywords: Performance, Causation, Quality, ISO 9001 Certification

Voluntary Leadership: Selection and Influence

International audienceIn social dilemmas, leading a team by making heroic efforts may prove costly, especially when the followers are not adequately motivated to make similar sacrifices. Attempting to shed light on what drives people to lead, we devise a two-stage public good experiment with endogenous timing. We show that leading by making generous contributions is widespread and relatively persistent. At least three motives explain this behavior. Some use leadership strategically to distill personal gains, with the expectation that others will respond by being at least as generous. Others are more altruistic, volunteering to lead even though this may come at a personal cost. Yet for another fraction of volunteers, a concern for maintaining a positive social image appears to be responsible. We also find that voluntary leaders are not necessarily more influential than randomly-chosen leaders

Agricultural management and plant selection interactively affect rhizosphere microbial community structure and nitrogen cycling.

BACKGROUND:Rhizosphere microbial communities are key regulators of plant performance, yet few studies have assessed the impact of different management approaches on the rhizosphere microbiomes of major crops. Rhizosphere microbial communities are shaped by interactions between agricultural management and host selection processes, but studies often consider these factors individually rather than in combination. We tested the impacts of management (M) and rhizosphere effects (R) on microbial community structure and co-occurrence networks of maize roots collected from long-term conventionally and organically managed maize-tomato agroecosystems. We also explored the interaction between these factors (M × R) and how it impacts rhizosphere microbial diversity and composition, differential abundance, indicator taxa, co-occurrence network structure, and microbial nitrogen-cycling processes. RESULTS:Host selection processes moderate the influence of agricultural management on rhizosphere microbial communities, although bacteria and fungi respond differently to plant selection and agricultural management. We found that plants recruit management-system-specific taxa and shift N-cycling pathways in the rhizosphere, distinguishing this soil compartment from bulk soil. Rhizosphere microbiomes from conventional and organic systems were more similar in diversity and network structure than communities from their respective bulk soils, and community composition was affected by both M and R effects. In contrast, fungal community composition was affected only by management, and network structure only by plant selection. Quantification of six nitrogen-cycling genes (nifH, amoA [bacterial and archaeal], nirK, nrfA, and nosZ) revealed that only nosZ abundance was affected by management and was higher in the organic system. CONCLUSIONS:Plant selection interacts with conventional and organic management practices to shape rhizosphere microbial community composition, co-occurrence patterns, and at least one nitrogen-cycling process. Reframing research priorities to better understand adaptive plant-microbe feedbacks and include roots as a significant moderating influence of management outcomes could help guide plant-oriented strategies to improve productivity and agroecosystem sustainability

Selection for cognitive control: a functional magnetic resonance imaging study on the selection of task-relevant information

The complex environment we live in makes it necessary to distinguish relevant from irrelevant information constantly and reliably. The aim of the present study was to investigate the neural substrate underlying the selection of task-relevant information. We devised a new paradigm in which participants had to switch between two different tasks that were instructed by task cues. The task cues had a relevant and an irrelevant cue dimension. In congruent trials, both cue dimensions indicated the same task; in incongruent trials, they indicated different tasks; and in neutral trials, only the relevant dimension indicated a task. By comparing trials in which both cue dimensions were informative (congruent and incongruent trials) with trials in which only the relevant dimension was informative (neutral trials), we were able to show that the lateral prefrontal cortex and a region in the intraparietal sulcus are involved in the selection of task-relevant information. Furthermore, the present paradigm allows the influence of the selected task and stimulus dimension to be investigated. No significant influence was found in the prefrontal cortex, indicating that this region serves a very abstract role in the selection of task-relevant information