Modelling price paths in on-line auctions: smoothing sparse and unevenly sampled curves by using semiparametric mixed models

Summary.  On-line auctions pose many challenges for the empirical researcher, one of which is the effective and reliable modelling of price paths. We propose a novel way of modelling price paths in eBay's on-line auctions by using functional data analysis. One of the practical challenges is that the functional objects are sampled only very sparsely and unevenly. Most approaches rely on smoothing to recover the underlying functional object from the data, which can be difficult if the data are irregularly distributed. We present a new approach that can overcome this challenge. The approach is based on the ideas of mixed models. Specifically, we propose a semiparametric mixed model with boosting to recover the functional object. As well as being able to handle sparse and unevenly distributed data, the model also results in conceptually more meaningful functional objects. In particular, we motivate our method within the framework of eBay's on-line auctions. On-line auctions produce monotonic increasing price curves that are often correlated across auctions. The semiparametric mixed model accounts for this correlation in a parsimonious way. It also manages to capture the underlying monotonic trend in the data without imposing model constraints. Our application shows that the resulting functional objects are conceptually more appealing. Moreover, when used to forecast the outcome of an on-line auction, our approach also results in more accurate price predictions compared with standard approaches. We illustrate our model on a set of 183 closed auctions for Palm M515 personal digital assistants.     

On (n-l)-wise and joint independence and normality of n Random variables: an example

An example is given of a vector of n random variables such that any (n-1)-dimensional subvector consists of n-1 independent standard normal variables. The whole vector however is neither independent nor normal.     

Deletions in random binary search trees: A story of errors

The usual assumptions for the average case analysis of binary search trees (BSTs) are random insertions and random deletions. If a BST is built by n random insertions the expected number of key comparisons necessary to access a node is 2 ln n+O(1). This well-known result is already contained in the first papers on such ‘random’ BSTs. However, if random insertions are intermixed with random deletions the analysis of the resulting BST seems to become more intricate. At least this is the impression one gets from the related publications since 1962, and it is quite appropriate to speak of a story of errors in this context, as will be seen in the present survey paper, giving an overview on this story.     

Implementierung von Coaching für Schulleitung und Schulaufsicht

Implementation of coaching for principals and supervisory school authorityGerman schools will have to improve the quality of their work to be able to compete on an international level. School development is also personnel development. Establishing professionalism of school executives (among others by coaching) plays a major role in this matter. First of all the author describes the implementation process of internal coaching for principals and thereafter for officers of the supervisory school authority. In doing so, he also points out problems that can arise during such a project. The evaluations of surveys prove the acceptance and importance of coaching for this target group. Finally, possible future coaching offers for school executives are addressed.     

Some Properties of the Hazard Function of the Two-Mutation Clonal Expansion Model

We discuss the hazard function of the two-mutation clonal expansion model with time-dependent parameters, with particular emphasis on identifiability of the parameters. We explicitly construct identifiable parameter combinations, and illustrate the properties of the hazard function under perturbations of the underlying biological parameters.     

The intersection of social protection and mobilities: a move towards a ‘Practical Utopia’ research agenda

Currently, a number of contributions in mobility studies are looking for fruitful intersections with other ‘adjacent’ approaches . In this spirit, our theoretical paper argues to study one particular aspect: the intersection of social protection and mobilities. Currently, the provision of social services in the ‘West’ is strongly entrenched within nation-state logics, which assume that clients’ immobility is a precondition of service delivery and that national citizenship is the desirable conditionality of gaining social rights. To overcome such a wide-spread conflation of social security with state security, we introduce the heuristic concept ‘social protection’. It allows social security to be imagined beyond a state-centric perspective and avoids the pitfalls of either a citizenship or a migration approach by taking on a mobility perspective. Thus, for scholars anchored in mobility studies we propose how to develop a social security perspective in a progressive way. For readers from other areas, e.g. citizenship, migration or social policy, we will show how a mobility perspective enriched by a No Border approach can overcome a narrow Western, statist and static perspective on social security. Our goal is to conceptually open up what we call a ‘practical utopia’ research agenda, one that expands our political horizons for future and present socialities.     

Forms of Capital and Habitus in the Decision to Go on Academic Exchange

Although Canadian universities have made internationalization a strategic priority, only few Canadian students currently participate in any international study opportunities. Drawing on interviews with Canadian students who were about to spend some of their undergraduate education abroad as exchange students, or had recently returned from a study exchange, we argue that despite the importance of relatively high costs associated with international study experiences, cultural factors play a more important role in enabling the decision to study abroad. Being habitually exposed to travel in one's family, and having been encouraged to travel independently as a teenager or young adult emerge as important precursors to our participants’ decision to take part in study exchanges. Although one needs to engage critically with university administrators’ current preoccupation with internationalization, being exposed to an extended time abroad has shown to have positive effects, especially for otherwise underprivileged students. Given the exploratory nature of this study, we conclude with suggestions for further research aimed at confirming our findings and developing policy.     

Was ist hier los?

In our prior article in this issue, we establish that although they are related, organizational and group dynamics are in fact very different. In this second article, we go on to discuss some of the challenges that this poses for consultants. We describe the interplay of factors affecting the development of the dynamics within an organization and explore approaches to acquiring practical knowledge about an organization’s dynamics. Finally, we ask how useful group dynamics skills are for providing consultancy services within and to organizations and consider where and how specific organizational dynamics skills can be acquired.     

中国人口的不确定性研究

文章对2000年人口普查所反映的生育水平、出生性别比和人口年龄结构等主要人口指标存在的不确定性进行了梳理,并运用概率人口预测方法系统讨论了中国未来人口变动趋势过程中相关人口变量的不确定性问题。结果显示,当前对中国生育率水平、出生性别比和低龄儿童人口数量的估计都存在很大的差异。这些差异直接影响中国人口目前和未来的不确定性。