Open Order Rescheduling in Job Shops with Demand Uncertainty: A Simulation Study

This paper addresses the problem of open order rescheduling in a job shop. Results are reported of experiments conducted in a simulation model of an 8-machine job shop. Four different order updating policies are examined. The due-date process is modeled to include several realistic features about the pattern of due-date variability. These features are parameterized and tested at several levels. Tardiness results indicate that open order rescheduling is beneficial only when allowances are loosely set. The results indicate that inventory performance is improved by order rescheduling, particularly in cases when due dates are revised to earlier times than originally forecast.     

ON ESTIMATING PARAMETERS FOR BETA DISTRIBUTIONS

The purpose of this paper is to comment on and give historical perspective to two methdologies for estimating parameters of beta distributions. Fielitz and Myers [3] [4] developed and advocated a methodology using the method of moments, while Romesburg [20] advocated a methodology usingthe method of maximum likelihood. However, what Fielitz and Myers presented as new research and suggested as an area needing further study is ground already trampled. The authors have prepared a graph to underline the superiority of the maximum likelihood method in fitting beta distributions.     

Modeling Correlated Discrete Uncertainties in Event Trees with Copulas

Modeling the dependence between uncertainties in decision and risk analyses is an important part of the problem structuring process. We focus on situations where correlated uncertainties are discrete, and extend the concept of the copula‐based approach for modeling correlated continuous uncertainties to the representation of correlated discrete uncertainties. This approach reduces the required number of probability assessments significantly compared to approaches requiring direct estimates of conditional probabilities. It also allows the use of multiple dependence measures, including product moment correlation, rank order correlation and tail dependence, and parametric families of copulas such as normal copulas, t‐copulas, and Archimedean copulas. This approach can be extended to model the dependence between discrete and continuous uncertainties in the same event tree.     

QMRA for Drinking Water: 2. The Effect of Pathogen Clustering in Single‐Hit Dose‐Response Models

Spatial and/or temporal clustering of pathogens will invalidate the commonly used assumption of Poisson‐distributed pathogen counts (doses) in quantitative microbial risk assessment. In this work, the theoretically predicted effect of spatial clustering in conventional “single‐hit” dose‐response models is investigated by employing the stuttering Poisson distribution, a very general family of count distributions that naturally models pathogen clustering and contains the Poisson and negative binomial distributions as special cases. The analysis is facilitated by formulating the dose‐response models in terms of probability generating functions. It is shown formally that the theoretical single‐hit risk obtained with a stuttering Poisson distribution is lower than that obtained with a Poisson distribution, assuming identical mean doses. A similar result holds for mixed Poisson distributions. Numerical examples indicate that the theoretical single‐hit risk is fairly insensitive to moderate clustering, though the effect tends to be more pronounced for low mean doses. Furthermore, using Jensen's inequality, an upper bound on risk is derived that tends to better approximate the exact theoretical single‐hit risk for highly overdispersed dose distributions. The bound holds with any dose distribution (characterized by its mean and zero inflation index) and any conditional dose‐response model that is concave in the dose variable. Its application is exemplified with published data from Norovirus feeding trials, for which some of the administered doses were prepared from an inoculum of aggregated viruses. The potential implications of clustering for dose‐response assessment as well as practical risk characterization are discussed.     

Urban hazards: spatial ecology and survivorship of a turtle in an expanding suburban environment

Urban Ecosystems - Urbanization poses a threat to long-lived vertebrates, particularly from road mortalities that can threaten population persistence. We studied movements, behavior and...     

‘Wasting precious time’: young men with Duchenne muscular dystrophy negotiate the transition to adulthood

Supporting young disabled people at the transition to adulthood has long proved to be a challenging task. Young men with Duchenne muscular dystrophy (DMD) have until recently died before they reached adulthood. Now they are living longer but with a corresponding lack of forethought about how they should be supported in adulthood. This study investigated in three regions of England what was happening for young men with DMD and how they were being supported. Semi-structured interviews were conducted with 37 young men (aged 15+) with DMD and 58 other family members. The findings suggested that too many young men had finished education and training and were at home during the day without meaningful activity. In part, the complicated nature of shifting expectations across the life-course had made planning for an adult life with DMD very challenging. This could be exacerbated by problematic assumptions and stubborn barriers.     

Leadership,information, and risk attitude: A game theoretic approach

This paper experimentally investigates how risk attitudes mitigate leadership effectiveness in a collective setting with projects that exhibit both free riding and coordination problems. We take two novel approaches: 1) the introduction of economic game theory to psychological studies of leadership, and 2) the application of the leadership ontology of Drath et al. (2008) as a cross-disciplinary integrative framework. Leadership here is focused on the presence or absence of direction, alignment, and commitment as well as antecedent beliefs and practices that are held within a collective (for us, our experimental participants). Our leadership context is stripped down to very minimal conditions: three group members, an investment decision, and the introduction of information regarding group members' attitudes toward risk. We find that the mere mention of risk attitude (whether risky or risk averse) undermines leadership effectiveness in mitigating free riding for our 420 experimental participants. Our study's primary implications lie in the application of game theory methodology to the psychological study of leadership, the introduction of relevant individual difference constructs to economic studies of leadership, and the advocation of the Drath et al. (2008) framework as a helpful integrative mechanism for interdisciplinary leadership research.     

An improved algorithm for the triples test

An algorithm, in the form of a Fortran subroutine TRIPLE, is given to compute statistics for the triples test for symmetry, The computational complexity of the algorithm is O(n²) which is an improvement over the straight for ward method, which is O(n³).