OR/MS methodology—A conceptual framework

Many authors deal, explicitly or implicitly, with OR/MS methodology and the term ‘methodology’ has gained such an inclusive connotation that it comes to denote nothing in particular. This has diluted the analytical precision of OR/MS ‘methodology’ as well as its practicability. Therefore an attempt to form a conceptual framework for OR/MS methodology will be presented. This framework consists of seven components addressing specific methodological aspects: each component has specific properties which distinguish it from the others; all of them together form a consistent hierarchy suitable to aid the successful development and implementation of OR/MS methods.     

Mitochondrial Dynamics in Cell Life and Death

Mitochondria are dynamic organelles that are able to change their morphology during cellular development and to meet the energetic needs of the cell, for example. During the past several years, knowledge about the molecular machinery involved in the control of mitochondrial shape has improved considerably. In August 2005, leading researchers in the field of mitochondrial dynamics met at the Venetian Institute of Molecular Medicine in Padova, Italy, to discuss novel and intriguing findings presented at the first workshop on "Mitochondrial Dynamics in Cell Life and Death." Some of the findings related to apoptosis and aging are described in this Perspective.     

On value-judgements and ethics in health technology assessment

The widespread application of technology in health care has imposed a broad range of challenges. The field of health technology assessment (HTA) is developed in order to face some of these challenges. However, this strategy has not been as successful as one could hope. One of the reasons for this is that social and ethical considerations have not been integrated in the HTA process. Nowadays however, such considerations have been included in many HTAs. Still, the conclusions and recommendations of the HTAs are not followed. The reason for this may be that the methods for integrating ethics for HTA are not sufficiently developed, or that they are not adequate. This article presents a supplementary approach to the ethical inquiry in HTA. It is argued that a value analysis is crucial in order to address the ethical issues of health care technology in a fruitful way and to make viable decisions about such technology.

Pooling operators with the marginalization property

In this paper, we consider the problem of combining a number of opinions which have been expressed as probability measures P₁, …, P_n, over some space. It is shown that a pooling formula which has the marginalization property of McConway (1981) must be of the form T = Σⁿ_i=₁W_i P_i + (1 - Σⁿ_i =₁W_i)Q, where Q is an arbitrary measure and W₁, …, W_n ϵ [—1,1] are weights such that| Σ_J Σ _j w_j | ≤ 1 for every subset J of {1, …, n}. If, in addition, T is required to preserve the independence of arbitrary events A and B whenever these events are independent under each P_i, then either T = P_i for some 1 ≤ i ≤ n or T = Q, in which case Q takes values in {0, l}.     

The two-sample t test versus satterthwaite's approximate f test

A comparison between the two-sample t test and Satterthwaite's approximate F test is made, assuming the choice between these two tests is based on a preliminary test on the variances. Exact formulas for the sizes and powers of the tests are derived. Sizes and powers are then calculated and compared for several situations.     

The Volatility of Realized Volatility

In recent years, with the availability of high-frequency financial market data modeling realized volatility has become a new and innovative research direction. The construction of “observable” or realized volatility series from intra-day transaction data and the use of standard time-series techniques has lead to promising strategies for modeling and predicting (daily) volatility. In this article, we show that the residuals of commonly used time-series models for realized volatility and logarithmic realized variance exhibit non-Gaussianity and volatility clustering. We propose extensions to explicitly account for these properties and assess their relevance for modeling and forecasting realized volatility. In an empirical application for S&P 500 index futures we show that allowing for time-varying volatility of realized volatility and logarithmic realized variance substantially improves the fit as well as predictive performance. Furthermore, the distributional assumption for residuals plays a crucial role in density forecasting.     

Sequential Pattern Analysis: A Statistical Investigation of Sequence Length and Support

In sequential pattern analysis, the frequency of patterns is evaluated by the support. While computed efficiently from large databases, we show that the support cannot be compared between different databases, since it is influenced by the actual sequence length distribution. Models for this sequence length distribution are surveyed. One of these models, the Good distribution, appears to be sufficiently flexible for practice. It is used to exemplify an approach for adjusting the relative support such that the resulting adjusted support values are better comparable between different databases. We illustrate our findings with texts from the bilingual FinDe corpus.     

The Interplay Between Adolescents' Friendships and the Exchange of Help: A Longitudinal Multiplex Social Network Study

The aim of this study was to unravel the interrelatedness of friendship and help, and to examine the characteristics of friendship and help networks. The effects of mutual versus one‐sided help relations on friendship initiation and maintenance, and vice versa, were examined. Friendship and help networks were analyzed (N = 953 students; 41 classrooms; M_age = 12.7). The results illustrate that friendship and help networks show some similarities, but only partly overlap and have distinct characteristics. Longitudinal multiplex social network analyses showed that mutual help was important for the maintenance of friendship, but not for the initiation of friendship. Further, particularly mutual friendships provided a context in which help took place. Implications of these findings are discussed.