ON THE INDEPENDENCE OF THE SAMPLE MEAN AND TRANSLATION-INVARIANT STATISTICS FOR MATRIX NORMAL DISTRIBUTIONS

We present an explicit characterization of the joint dependency structure of an n×p matrix normal random matrix such that the p-dimensional sample mean vector is independent of all translation invariant statistics.     

Optimal Times to Contact Sample Households

Finding a suitable respondent at home is an essential and expensivecomponent of a household survey. This article reports on theresults of a study of the probabilities of finding someone aged14 or older at home and discusses the application of such datato survey design and budgeting.     

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.     

An evaluation of the tandem configuration automated guided vehicle system

The performance of a tandem automated guided vehicle (AGV) system is compared with that of conventional AGV track systems. In the tandem system the track is divided into non-overlapping, single-vehicle closed loops. Using simulation it is shown that, because of trips requiring delivery across loops, the tandem system has a higher expected travel time per load and thus a greater average time in system. When the loads are delivered within the original loop only, the tandem system has a lower average time in system than in conventional systems, although the difference is modest. Track layout appears to influence the average time in system much more than docs the dispatching rule invoked. The success of the tandem system is highly dependent on approximately even AGV utilization, which may be difficult to obtain in practice.     

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.     

ABORTION AND CRIME: UNWANTED CHILDREN AND OUT-OF-WEDLOCK BIRTHS

Legalizing abortion can either increase or decrease investments in children's human capital. This article finds that abortion increases the number of out-of-wedlock births. Using data that more directly links the criminal with age when the crime was committed, not age when arrested, and fixing the assumption in previous research that no abortions took place prior to the Roe v. Wade decision in the 45 states affected by that decision, we find consistent significant evidence that legalizing abortions increased murders by over 7%. Linear estimates indicate that legalization increased total annual victimization costs by at least $3.2 billion. ( JEL K42, K14, J24)     

Job queues, discrimination, and affirmative action

If employers have different Becker-type discrimination coefficients for different demographic groups, then the implementation of affirmative action may have a differential impact on those groups. We estimate two vector autoregressive models of the U.S. economy, including the unemployment rates of four demographic groups. We find that a job queue existed before the implementation of affirmative action and that affirmative action changed the ordering of the job queue in manner that had a negative impact on nonwhite males. We find evidence that affirmative action may have increased the unemployment rate of nonwhite males by increasing their duration of unemployment.     

Television Coverage of State Government

An empirical analysis of 3,334 state and local television newsstories in 10 cities clarifies the reluctance of commercialtelevision newscasters to cover state government. Contrary towhat many television news directors assert, television's minimalattention to state government cannot be attributed to an allegednegative relationship between state government and newscastratings. Rather, it appears to stem from the high cost of obtainingfilmed coverage of state government in a timely fashion.