A simple GMM estimator for the semi-parametric mixed proportional hazard model

Ridder and Woutersen (2003) have shown that under a weak condition on the baseline hazard, there exist root-N consistent estimators of the parameters in a semiparametric Mixed Proportional Hazard model with a parametric baseline hazard and unspeci�ed distribution of the unobserved heterogeneity. We extend the Linear Rank Estimator (LRE) of Tsiatis (1990) and Robins and Tsiatis (1991) to this class of models. The optimal LRE is a two-step estimator. We propose a simple one-step estimator that is close to optimal if there is no unobserved heterogeneity. The e¢ ciency gain associated with the optimal LRE increases with the degree of unobserved heterogeneity.

Electronic purchasing: determining the optimal roll-out strategy

Electronic purchasing (EP), also known as electronic ordering through catalogs is the most established form of e-procurement nowadays, yet still in its infancy. Theoretically, changing from the "traditional" way of purchasing to EP can lead to huge cost savings. However the implementation (roll-out) of EP including many commodity groups and many departments is a large and costly task. In addition, not much experience on good roll-out strategies is available yet. This paper contributes to the solution of this problem, by presenting a mathematical model for determining the optimal EP roll-out strategy into an organisation based on maximisation of the cost savings. Results from this model suggest that the optimal order of commodity groups and departments for which EP is implemented can contribute considerably to the possible savings that can be realised and is therefore an important factor for a successful implementation strategy of EP

Unobserved Heterogeneity in Multiple-Spell Multiple-States Duration Models

In survival analysis a large literature using frailty models, or models with unobserved heterogeneity, exist. In the growing literate on multiple spell multiple states duration models, or multistate models, modeling this issue is only at its infant phase. Ignoring unobserved heteogeneity can, however, produce incorrect results. This paper presents how unobserved heterogeneity can be incorporated into multistate models, with an emphasis on semi-Markov multistate models with a mixed proportional hazard structure. First, the aspects of frailty modeling in univariate (proportional hazard, Cox) duration models are addressed and some important models with unobserved heterogeneity are discussed. Second, the domain is extended to modeling of parallel/clustered multivariate duration data with unobserved heterogeneity. The implications of choosing shared or correlated unobserved heterogeneity is highlighted. The relevant differences with recurrent events data is covered next. They include the choice of the time scale and risk set which both have important implications for the way unobserved heterogeneity influence the model. Multistate duration models can have both parallel and recurrent events. Incorporating unobserved heterogeneity in multistate models, therefore, brings all the previously addressed issues together. Although some estimation procedures are covered the emphasis is on conceptual issues. The importance of including unobserved heterogeneity in multistate duration models is illustrated with data on labour market and migration dynamics of recent immigrants to The Netherlands.multiple spell multiple state duration, mixed proportional hazard, multistate model, unobserved heterogeneity, frailty

Labour Market Status and Migration Dynamics

In this empirical paper we assess how labour market transitions and out- and repeated migration of immigrants are interrelated. We estimate a multi-state multiple spell competing risks model with four states: employed, unemployed receiving benefits, out-of-the-labour market (no benefits) and abroad. For the analysis we use data on recent labour immigrants to The Netherlands, which implies that all migrants are (self)-employed at the time of arrival. We find that many migrants leave the country after a period of no-income. Employment characteristics and the country of origin play an important role in explaining the dynamics. Microsimulations of synthetic cohorts reveal that many migrants experience unemployment spells, but ten years after arrival only a few are unemployed. Scenarios based on microsimulation indicate that the Credit Crunch will not only increase the unemployment among migrants but also departure from the country. Scenarios also indicate that an increase in the number of migrants from the EU accession countries will lead to higher labour market and migration dynamics. Finally, based on microsimulation we do not expect that the recent simplification of the entry of high income migrants will have a lasting effect, as many of those migrants leave fast.migration dynamics, labour market transitions, competing risks, immigrant assimilation

Optimal number of tenders in practice

In this paper we consider what is the current practice when a purchaser has to decide on the number of bids he wants to receive in a tender procedure. Furthermore we look to what extent a formal model, the so-called ETQ-model, together with a DSS can be useful in facilitating and improving that decision. The validity of the model assumptions is analyzed with empirical evidence from over thirty cases. Furthermore, a few interviews were conducted with tactical purchasers to gain insight into the practical applicability of the model and the DSS and identify specific directions for further improvement

Holographic QCD in the Veneziano limit at finite Magnetic Field and Chemical Potential

We investigate the phase diagram of QCD-like gauge theories at strong coupling at finite magnetic field $B$, temperature $T$ and baryon chemical potential $\mu$ using the improved holographic QCD model including the full backreaction of the quarks in the plasma. In addition to the phase diagram we study the behavior of the quark condensate as a function of $T$, $B$ and $\mu$ and discuss the fate of (inverse) magnetic catalysis at finite $\mu$. In particular we observe that inverse magnetic catalysis exists only for small values of the baryon chemical potential. The speed of sound in this holographic quark-gluon plasma exhibits interesting dependence on the thermodynamic parameters.Comment: 7 pages, 6 figure

A Simple GMM Estimator for the Semi-Parametric Mixed Proportional Hazard Model

Ridder and Woutersen (2003) have shown that under a weak condition on the baseline hazard there exist root-N consistent estimators of the parameters in a semiparametric Mixed Proportional Hazard model with a parametric baseline hazard and unspecified distribution of the unobserved heterogeneity. We extend the Linear Rank Estimator (LRE) of Tsiatis (1990) and Robins and Tsiatis (1991) to this class of models. The optimal LRE is a two-step estimator. We propose a simple first-step estimator that is close to optimal if there is no unobserved heterogeneity. The efficiency gain associated with the optimal LRE increases with the degree of unobserved heterogeneity.mixed proportional hazard, linear rank estimation, counting process

A model for assessing the impact of electronic procurement forms

This paper aims to contribute to the development of a model for assessing the direct and indirect impact of various forms of electronic procurement (EP) on a firm¿s integral purchasing (-related) costs. The model builds on existing classifications of purchasing costs and benefits and is illustrated by means of a number of empirical cases

Modeling migration dynamics of immigrants: the case of the Netherlands

In this paper we analyze the demographic factors that influence the migration dynamics of recent immigrants to The Netherlands. We show how we can allow for both permanent and temporary migrants. Based on data from Statistics Netherlands we analyze both the departure and the return from abroad for recent non-Dutch immigrants to The Netherlands. Results disclose differences among migrants by migration motive and by country of origin and lend support to our analytical framework. Combining both models, for departure and returning, provides the probability that a specific migrant ends-up in The Netherlands. It also yields a framework for predicting the migration dynamics over the life-cycle. We can conclude that for a complete view of the migration dynamics it is important to allow for both permanent (stayers) migrants and temporary (movers) migrants and that return from abroad should not be neglected

The impact of labour market dynamics on the return-migration of immigrants

Using administrative panel data on the entire population of new labour immigrants to The Netherlands, we estimate the effects of individual labour market spells on immigration durations using the “timing-of-events” method. The model allows for correlated unobserved heterogeneity across migration, unemployment and employment processes. We find that unemployment spells increase return probabilities for all immigrant groups, while re-employment spells typically delay returns