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Setups and effective capacity: the impact of lot sizing techniques in an MRP environment

This study investigates how lot sizing techniques influence the profit performance, inventory level, and order lardiness of an assembly job shop controlled by MRP. Four single-level lot sizing techniques are compared by simulation analysis under two levels of master schedule instability and two levels of end item demand. A second analysis investigates the influence of a multilevel lot sizing technique, the generalized constrained-K (GCK) cost modification, on the four single-level techniques at low demand and low nervousness. The analyses reveal a previously unreported phenomenon. Given the same inventory costs, the single-level lot sizing techniques generate substantially different average batch sizes. The lot sizing techniques maintain the following order of increasing average batch size (and decreasing total setup time):

economic order quantity (EOQ)

period order quantity (POQ)

least total cost (LTC)

Silver-Meal heuristic (SML)

The causes for different average batch sizes among the lot sizing techniques are analysed and explained. Demand lumpiness, inherent in multilevel manufacturing systems controlled by MRP, is found to be a major factor. The number of setups each lot sizing technique generates is the primary determinant of profit performance, inventory level, and order tardiness. EOQ, a fixed order quantity technique, is less sensitive to nervousness than the discrete lot sizing techniques. EOQ_, however, generates the smallest average batch size, and, therefore, the most setups. Since setups consume capacity, EOQ, is more sensitive to higher demand. SML generates the largest average batch sizes, and is, therefore, less sensitive to increased demand. At low demand, the other lot sizing techniques perform better on all criteria. They generate smaller batches and, therefore, shorter actual lead times. The GCK cost modification increases the average batch size generated by each lot sizing technique. GCK improves the profit and customer service level of EOQ the lot sizing technique with the smallest batches. GCK causes the other lot sizing techniques to generate excessively large batches and, therefore, excessively long actual lead times.     

Evaluation of a class-based storage scheduling technique applied to dual-shuttle automated storage and retrieval systems

Research to improve the performance of automated storage and retrieval systems has concentrated on developing more effective system designs or scheduling control programs to reduce the time a shuttle spends travelling in a rack. Sarker et al. (1991) showed that a dual-shuttle AS/RS operating under a nearest-neighbour scheduling model would significantly improve system throughput performance over a single-load shuttle system. The study was limited in that it only considered dual-shuttle improvements under the nearest-neighbour model. This research exploits the benefits of a dual-shuttle system by integrating the technology with a more effective scheduling technique, the class-based storage model, for reducing the shuttle travel time. The dual-shuttle, class-based storage model produced a significant reduction in shuttle travel time and thus higher throughput results over the dual-shuttle, nearest-neighbour model. This study further supports the premise that a dual-capacity shuttle is an effective way of improving the productivity of an automated storage and retrieval system.     

On Bayesian learning from Bernoulli observations

We provide a reason for Bayesian updating, in the Bernoulli case, even when it is assumed that observations are independent and identically distributed with a fixed but unknown parameter _θ0${\theta }_0$. The motivation relies on the use of loss functions and asymptotics. Such a justification is important due to the recent interest and focus on Bayesian consistency which indeed assumes that the observations are independent and identically distributed rather than being conditionally independent with joint distribution depending on the choice of prior.     

A Bayesian nonparametric estimator based on left censored data

Summary This paper introduces a Bayesian nonparametric estimator for an unknown distribution function based on left censored observations. Hjort (1990)/Lo (1993) introduced Bayesian nonparametric estimators derived from beta/beta-neutral processes which allow for right censoring. These processes are taken as priors from the class ofneutral to the right processes (Doksum, 1974). The Kaplan-Meier nonparametric product limit estimator can be obtained from these Bayesian nonparametric estimators in the limiting case of a vague prior. The present paper introduces what can be seen as the correspondingleft beta/beta-neutral process prior which allow for left censoring. The Bayesian nonparametyric estimator is obtained as in the corresponding product limit estimator based on left censored data.     

Class,work and family in women's lives

Interviews with white, working professional and working class mothers suggest that these women share values about work, family and on-the-job sociality. Respondents draw from a common value system, and many of the statements voiced by one group are echoed in the other. Work and motherhood are important paths to the development of individual identity for both working class and professional women. Differences in jobs, however, lead to differences in the actualization of values in their lives. Professional women are pulled in two directions and face continuing dilemmas in balancing careers which extract strong commitments from them and the demands of motherhood. Working class women's jobs do not require the same intensity of commitment, and they find it easier to meet their families' demands on their time. But conceptualizing their work as meaningful becomes more difficult than it is for professional women. Sociality is the third area of concern. Women in this study emphasized sociality because it allowed them to complete their work more effectively. It also served expressive functions; some women made friends at work. Some de-emphasized sociality if it interfered with their work identities. The importance of sociality was not necessarily dependent upon respondents' class location.I would like to thank Harold Bershady, Charles Bosk, and Robin Leidner for their comments and encouragement during the research and writing of this project. I would also like to thank M. E. Hughes, several anonymous reviewers, andQualitative Sociology for their helpful comments on earlier version of this article.     

The effect of public policies on recent Swedish fertility behavior

In the literature the recent upsurge in period birth rates is seen as evidence of a pronatalist effect of Sweden's extensive social insurance programs. Yet, these explanations can not account for the downturn in birth rates in the 1970s, the delay in childbearing, and the constancy of cohort birth rates which characterize recent Swedish fertility behavior. To summarize the effect of Sweden's economic and policy environment on the observed fertility patterns, I use a neoclassical economic framework to develop the shadow price of fertility. Although strong simplifying assumptions are imposed, the estimated price series exhibit a negative relationship with period fertility rates and the change in the estimated relative prices of fertility over the life cycle lend modest support for the delayed childbearing.The first draft of this paper was written while I was a National Fellow at the Hoover Institution. The research was supported by grants number HD-19226 and HD-28685 from the National Institute of Child Health and Development. I thank Glen Cain, John Kerman, Tom MaCurdy, Duncan Thomas, Michael Tilkin, and seminar participants at University of Illinois, Hoover Institution, and University of Wisconsin for useful comments.     

The prevalence of problem and pathological gambling: A critical analysis

Prevalence of pathological gambling refers to the percentage of cases of pathological gambling occurring in the community at a given time. Prevalence studies conducted in different principalities throughout the world are reviewed, and it is found that none of them conforms to this definition of prevalence. The major error in all but the most recent surveys conducted is identified as the use of questions which ask whether gambling-related problems have ever occurred rather than whether they are currently occurring. This error will lead to an over-estimation of the prevalence of pathological gambling in society. The second major error identified in nearly all studies involves the accuracy of the screens being used to assess whether or not an individual is a pathological gambler. Concerns about the efficiency of the South Oaks Gambling Screen have not yet been satisfactorily resolved. None-the-less, the widespread use of the South Oaks Gambling Screen has made a valuable contribution to international comparisons of prevalence studies. Future work that explores the emerging relationship between levels of personal expenditure on gambling, types of gambling product and gambling-related problems are recommended.     

Productivity indices and survival rate estimates from MAPS,a continent-wide programme of constant-effort mist-netting in North America

The Monitoring Avian Productivity and Survivorship (MAPS) programme is a cooperative effort to provide annual regional indices of adult population size and post-fledging productivity and estimates of adult survival rates from data pooled from a network of constant-effort mist-netting stations across North America. This paper provides an overview of the field and analytical methods currently employed by MAPS, a discussion of the assumptions underlying the use of these techniques, and a discussion of the validity of some of these assumptions based on data gathered during the first 5 years (1989-1993) of the programme, during which time it grew from 17 to 227 stations. Ageand species-specific differences in dispersal characteristics are important factors affecting the usefulness of the indices of adult population size and productivity derived from MAPS data. The presence of transients, heterogeneous capture probabilities among stations, and the large sample sizes required by models to deal effectively with these two considerations are important factors affecting the accuracy and precision of survival rate estimates derived from MAPS data. Important results from the first 5 years of MAPS are: (1) indices of adult population size derived from MAPS mist-netting data correlated well with analogous indices derived from point-count data collected at MAPS stations; (2) annual changes in productivity indices generated by MAPS were similar to analogous changes documented by direct nest monitoring and were generally as expected when compared to annual changes in weather during the breeding season; and (3) a model using between-year recaptures in Cormack-Jolly-Seber (CJS) mark-recapture analyses to estimate the proportion of residents among unmarked birds was found, for most tropical-wintering species sampled, to provide a better fit with the available data and more realistic and precise estimates of annual survival rates of resident birds than did standard CJS mark-recapture analyses. A detailed review of the statistical characteristics of MAPS data and a thorough evaluation of the field and analytical methods used in the MAPS programme are currently under way.