Some statistical issues in weather experimentation

A number of topics of statistical methodology in weather modification are discussed. The time sequence of unit definition, classification and randomization is shown to affect the types of units that can be used validly, and this casts doubt on the value of blocking. Re-randomization (permutation) tests are recommended as the only reliable method of confirmatory inference for weather experiments. Some aspects of such tests are examined, including a procedure for multiple comparisons. The plague of multiplicity of tests is discussed and warned against. Doubts about cumulative evaluations of "all" experiment's are expressed. A case is argued for examination of some non-randomized seeding operations. Consid¬ering the dearth of randomized data, it is argued that careful evaluation of seeding operations should be undertaken.     

On the explosion rate of maximum-bias functions

The maxbias function B_T() contains much information about the robustness properties of the estimate T. This function satisfies B_T(0)=0 and B_T()<for all 0<<whereis the breakdown point of T. Hampel (1974)pioneered the study of the limiting behaviour of B_T(?) where ? → 0. He computed and optimized the rate γ at which B_T(?) approaches 0 when ? → 0. This rate is now called the contamination sensitivity of T, and constitutes one of the cornerstones of the theory of robustness. We show that much can also be learned from the study of the limiting behaviour of B_T(?) when ? → ?*. A new robustness measure, called the relative explosion rate, can be obtained by studying the limiting relative maxbias behaviour of two extimates when approaches their common breakdown point ?*. Like the contamination sensitivity, the relative explosion rate can be readily derived from the estimate's score function. General formulae are given for M-estimates of scale and S-, MM- and τ-estimates of regression. We also show that the maxbias behaviour for large ? is largely determined by the curvature of the estimate's score function near zero. This motivates our definition and study of the local order of a score function.     

Life Paths of Migrants: A Sequence Analysis of Polish Migrants’ Family Life Trajectories

After Poland’s accession to the European Union in 2004, Polish migration within Europe has increased considerably. Still, little is known about this large and growing group of immigrants. Using unique panel data from the Dutch population registers, we first apply sequence analysis to explore and describe the variety of family life courses among young adult (aged 20–30 at migration) Polish migrants in the Netherlands. Each individual family life career is followed for a 5-year period on a monthly time base. Optimal matching analysis is applied to compute (dis)similarities between individuals, in order to cluster into predominant life paths. Second, a series of logistic regression analyses were carried out to determine the main factors of importance on the different family life paths. Findings showed a strong impact of the partner’s origin on the type of union and likelihood of return migration. Unemployment was found to be another important trigger of return migration. With regard to gender differences, we found that men are more likely to remain single and to leave the Netherlands at some point, while women more often cohabit and tend to stay.     

Lot Splitting in Unbalanced Production Systems*

This research investigates the impact of lot splitting in unbalanced production systems, under a variety of experimental conditions. Scheduling policies specifically designed for use in the presence of a long-term bottleneck, a condition frequently encountered in practice, are developed and tested. Results indicate that when steps are taken at nonbottleneck work centers to capitalize on capacity imbalances through increasing the number of setups and, hence, the variety of products produced, shop effectiveness is improved. The results also indicate that scheduling policies that tend to increase the size of the average process batch retard the overlapping of operations, which is critical to the success of the lot-splitting methodology in reducing flow time. Finally, it is shown that increasing capacity at nonbottleneck work centers along with implementation of effectiveness-oriented scheduling polices leads to improved shop performance.     

The longitudinal development of students' well-being in adolescence: The role of perceived teacher autonomy support

Although a decline in adolescents' well-being has repeatedly been reported, longitudinal evidence for this development is rare and time-varying factors like teacher autonomy support that could be associated with this trend have sparsely been investigated. Therefore, the present study examined how the temporal development of perceived autonomy support from their German language arts teachers is related to changes in four different facets of students' well-being. Longitudinal data from 3446 adolescents from Germany (N_Schools = 178) on five measurement points (Grades 5–9) were analyzed using latent growth curve models. Satisfaction with school, enjoyment of school, and self-rated health decreased over time, while social integration remained stable. Perceived teacher autonomy support also declined between Grades 5 and 9. Furthermore, baseline levels of perceived teacher autonomy support and facets of well-being were positively related. Finally and most importantly, our results indicated that changes in perceived teacher autonomy support were positively associated with the development of satisfaction with school, enjoyment of school, and self-rated health, but not social integration. The findings suggest that perceived teacher autonomy support plays an important role in the development of students' well-being in adolescence.     

An Evaluation of Flexible Workday Policies in Job Shops*

Job shops have long faced pressures for improvement in a challenging and volatile environment. Today's trends of global competition and shortening of product life cycles suggest that both the challenges and the intensity of market volatility will only increase. Consequently, the study of tactics for maximizing the flexibility and responsiveness of a job shop is important. Indeed, there is a significant body of literature that has produced guidelines on when and how to deploy tactics such as alternate routings for jobs and transfers of cross‐trained workers between machines. In this paper we consider a different tactic by adjusting the length of workdays. Hours in excess of a 40‐hour week are exchanged for compensatory time off at time and a half, and the total amount of accrued compensatory time is limited to no more than 160 hours in accordance with pending legislation. We propose several simple flexible workday policies that are based on an input/output control approach and investigate their performance in a simulated job shop. We find significant gains in performance over a fixed schedule of eight hours per day. Our results also provide insights into the selection of policy parameters.