An empirical analysis of job shop dispatch rule selection

Job shop scheduling usually includes the process of selecting dispatch rules for loading shops with work. Traditionally, dispatch rules have been formed on the basis of processing time, operating time, or queueing order. A job shop scheduling model was developed to include external factors (such as due dates), internal factors (e.g., capacity), as well as influence factors (e.g., job status). Based on the model developed in this report a survey of industrial engineers, shop foremen, and production control supervisors was undertaken to determine what dispatch rules experienced job shop schedulers would select and if the selection process could be influenced by schedule conditions (status) or other organizational factors. Results suggest that schedulers may be influenced by other factors. This article suggests a model for further research with respect to job shop scheduling.     

Use of the net present value criterion in a random job shop where early shipments are forbidden

Both practitioners and researchers in the field of Operations Management have suggested that shop scheduling should be an integral component in both the strategic and tactical plans for an organization's assets. This paper examines the use of an accepted measure of return on assets, net present value (NPV), in a simulated shop scheduling environment where early shipment of jobs before their due dates is forbidden. In addition, early shipment of raw materials to the shop is also forbidden. This shop environment is consistent with the prevalent practice in industry of accepting orders only on a just-in-time basis to reduce purchased parts inventories. The NPV measure provides a means of balancing a variety of performance criteria that have been treated as separate objectives previously, including work-in-process inventory, finished goods inventory, mean flow time and mean tardiness, while also providing a means of measuring monetarily the value of various shop scheduling approaches.The NPV performance of priority scheduling rules and order release policies is measured in this research through the simulation of a random job shop under a variety of environmental conditions. It is found in a comparison of priority rules that use time-based information with those that use job value information that the Critical Ratio rule provides higher average performance than the three other rules used in the study. However, in some situations that are consistent with JIT practice, value-based priority rules also perform well. The use of a mechanism for delaying the release of jobs to each work center in the shop provided higher average NPV when shop utilization was set at a low level of 80%, while immediate release of work upon its arrival to the shop provided superior performance at a higher shop utilization level of 94%. While JIT materials delivery and costing yields higher NPV, it did not alter the relative ranking of priority rule/release policy combinations. In addition, it was found that environmental factors, including average job length, average number of tasks per job and level of tardiness penalty, resulted in greater variations in NPV performance than the institution of a JIT raw materials delivery policy.     

码头间集装箱拖箱业务调度系统研究

分析同一港口集装箱码头间拖箱业务产生原因的基础上,根据上海港（案例港）的实地调研数据,采用Dijkstra算法和模糊综合评判法,建立码头间集装箱拖箱业务调度模型及调度系统。用C＋＋11语言,编写了适用于港口的数字模拟平台,通过仿真,验证了设计的新调度模型比传统人工方式更有效率,为港口运营提供了切实可行的系统支持。     

A note on workload-dependent due date assignment rules

For many production systems, delivery performance relative to promised job due dates is a critical evaluation criterion. Delivery performance is affected by the way in which work is dispatched on the shop floor, and also by the way the job due dates are assigned to begin with. This pape shows how information regarding congestion levels on the shop floor can be used to assign due dates to arriving jobs in such a way that the mean tardiness of jobs is decreased without increasing the average length of the promised delivery lead times.Baker and Bertrand suggested a modification of the Total Work (TWK) rule for assigning job due dates which adjusts the job flow allowance according to the level of congestion in the shop. Their method gives longer flow allowances to jobs which arrive when the system is congested. Although their modified TWK rule results in lower mean tardiness in many settings, it also generally results in a higher proportion of jobs tardy.This paper presents an alternative modification of the TWK rule which, in most cases, provides mean tardiness as low as or lower than Baker and Bertrand's rule and also results in a lower proportion of jobs tardy. The alternative rule suggested here still results in higher proportion of tardy jobs than the non-workload adjusted rule in most settings, but suggestions are made for how this problem might be addressed.     

Load-limited order release in job shop scheduling systems

This report examines the practice of using work load limits to control the release of orders to a job shop. Load limits function in the following general way. Whenever the inventory of work at a work center exceeds some critical value (its “load limit”), further release of orders which are routed to that work center are blocked from entering the shop. After the inventory is “worked off,” release of work to the shop gateways is again permitted. Load-limited order release is intuitively appealing because it appears to be a method for reducing system inventory and flow times. The practice of load limiting order release is becoming popularized by some of the recent production planning software products now on the market. A notable example is OPT. In this report, analytical results for an M/M/1 queueing model, along with existing simulation studies of multi-machine job shops are interpreted to form a theory about the effects of using load limits.The major finding here is the proposition that system flow time, inventory, and order tardiness all deteriorate to the extent that load limits introduce idle time into the schedule. Based on the arguments presented here, a very cautious approach toward the use of input control schemes for anywhere but gateway work centers would be advised. The conclusions drawn here are to a great extent arrived at by interpreting the research results of others, so there is a clear need for further research which tests these assertions in a more direct and controlled way.     

The effects of input control in a simple scheduling model

Input control is a generic procedure for smoothing production workload by delaying work during intervals of heavy load. While input control techniques have several practical benefits, they also have an inherent disadvantage. By restricting the set of jobs available for scheduling, an input control procedure removes some of the scheduling options that would otherwise be available. This paper examines the impact of such a procedure in a simple simulation model.The simulation model represents a production shop in a simplified way, as a single machine, but the production control system has three distinct parts. The first part assigns due-dates to customer orders, taking into consideration the size of each job and the workload in the shop. The second part is a job releasing rule that implements input control. The third part is a priority dispatching procedure that is aimed at meeting due-dates. By representing this three-part control system the model provides an opportunity to explore the interdependence between input control and other control procedures.Reinforcing previous research, the simulation experiments confirm that modified due-date priorities perform more effectively than other basic priority rules when performance is measured by average tardiness. Moreover, the experiments indicate that performance under the modified due-date regime is never improved by the use of input control. On the other hand, with dispatching rules that rely on shortest-first or critical ratio priorities, the experiments indicate that input control is sometimes advantageous. The effects of input control on scheduling performance thus appear to be somewhat complicated, and further experiments were designed to explore some of the relationships involved. The principal finding, however, provides a warning that input control can be counterproductive.     

Priority dispatching with operation due dates in a job shop

This paper investigates the effects on job shop performance of setting and attempting to enforce operation due dates for jobs. It does so by comparing via simulation the performance of priority dispatching rules when employed to enforce operation due dates versus job due dates. The results indicate that operation due dates improve every conventional measure of job shop performance.     

Employed and Unemployed Job Seekers and the Business Cycle

The job search literature suggests that on‐the‐job search reduces the probability of un employed people finding jobs. However, there is little evidence that employed and unemployed job seekers are similar or apply for the same jobs. We compare employed and unemployed job seekers in their individual characteristics, preferences over working hours, job‐search strategies and employment histories, and identify how differences vary over the business cycle. We find systematic differences which persist over the business cycle. Our results are consistent with a segmented labour market in which employed and unemployed job seekers are unlikely to directly compete with each other for jobs.     

Application of work center control rules in a job shop environment

Work center control rules, defined as a combination of job dispatch rules and short-term work center capacity adjustments, are analyzed using queueing theory. Promising rules are evaluated with a job shop simulation model. Simulations comparing work center control rules to the critical ratio rule for job dispatching indicate that work center control can increase performance to customer due date while simultaneously reducing average work in process inventory. The work center control rules are easily implemented by shops currently using input/output control and daily dispatch lists.     

The use of workload information to control job lateness in controlled and uncontrolled release production systems

The performance of a workload dependent scheduling and due date assignment rule is investigated. The rule uses time-phased workload information and time-phased capacity information. Performance comparisons are made with a rule that uses only time-aggregated workload information. The performance for two methods of releasing jobs to the shop is investigated: an uncontrolled method where the jobs are released at random, and a controlled method where jobs are released to maintain a specific workload norm. Computer simulation is used as the research tool. The mean and standard deviation of lateness are used as performance measures.The results indicate that the use of time-phased workload information may decrease the variance of the lateness, as compared with the use of time-aggregated workload information only. However, the magnitude of the effect depends on the type of sequencing rule used. Furthermore, if time-phased information is used, control of the mean lateness requires that the amount of capacity available for loading used in the loading procedure differs from the amount of capacity actually available for production. It is shown that by selecting adequate parameter values, both a constant mean lateness and a small variance of lateness can be obtained with this type of assignment rule.These results are valid for both types of release situations. So even in the case where the workload of the shop is under strict control, and the mean operation flow time does not, therefore, vary, the use of time-phased workload information can decrease the variance of the lateness.     

Sequencing rules and due date setting procedures in flow line cells with family setups

The purpose of this study is to investigate due date setting procedures and dispatching decisions in a flow line cell with family setups. In this environment, setups are not required when switching from a job in a given family to a job in the same family. However, switching from a job in one family to a job in another family requires a setup. Family setups in this shop are sequence independent. The dispatching decisions in this shop are threefold: (1) when should the decision to switch from one part family to another be made; (2) once the decision to switch families is made, how should the next part family be chosen (next family decision); and (3) how should the jobs within a family be prioritized (next job decision)? If the decision to switch classes can only be made after the current family is exhausted, the rule is called a class exhaustion rule. Otherwise the rule is a truncated rule. The results indicate that the due date setting procedure has a major impact on how dispatching should be performed in the shop. The family exhaustion procedure using the APT next family rule and the SPT next job rule is the best performer for mean flow time. When setup times are long, the SEQ due date rule using the family exhaustion procedure with the FCFS next family and the EDD next job rules performed well for due date criteria. When setup times are short, the EDD/T, Sawicki truncation rule and the family exhaustion rules performed well for due date criteria.     

Order review/release in the absence of adherence to formal scheduling policies

In many plants, the performance of shop floor workers is measured by accounting-based productivity criteria. Such systems encourage workers to maximize their individual performance, often at the expense of total shop performance. One such company, Union Switch & Signal, a manufacturer of railroad equipment, has decided to increase finished goods inventory in an effort to counteract poor due date performance. Management at Union Switch & Signal feel that workers not following priorities contribute significantly to this poor performance. It has been suggested that the controlled release of jobs into the shop, i.e., Order Review/Release (ORR), may provide the operations manager a vehicle for enforcing job priorities when formal dispatching rules are not strictly followed by workers. In this study, two ORR methodologies are studied in regards to their ability to offset the dysfunctional behavior by workers who seek to maximize their own individual productivity. This type of behavior was captured by simulating the phenomenon of `cherry picking'. Cherry picking occurs when a job is selected for processing based not on its formal priority but on the difference in standard allowable processing time and actual processing time. Results suggest that at least one ORR methodology is able to reduce the difference in resulting labor productivity while improving overall shop performance.     

Spatial competition between shopping centers

We study competition between two shopping centers that sell the same set of goods and are located at the extremes of a linear city, without restricting consumers to make all their purchases at a single place. In the case of competition between a shopping mall (set of independent single-product shops) and a department store (single multiproduct shop), we find that: if the number of goods is low, all consumers shop at a single place; if it is moderately high, some consumers travel to both shopping centers to buy each good where it is cheaper (a single good is cheaper at the shopping mall). The shops at the mall, taken together, obtain a lower profit than the department store. Nevertheless, two shopping malls should be expected to appear endogenously.     

基于知识管理的车间调度规划

在以销定产的生产模式下,为了充分利用制造资源,提升制造企业核心竞争力,根据制造车间生产特点,提出了制造车间生产抽象化模型。将车间资源进行了分类,根据制造车间特点归纳出调度时应服从的原则,指出调度应服从的整体经济原则。对制造车间agent建立与调度优化方法,构建调度数据采集和网络做了分析。为功能各异、数据结构不同的车间资源调度的建模提供参考。     

动态集成制造系统环境下车间调度代理协商机制的研究

引入动态集成制造系统概念及其系统的构成，介绍动态集成系统的代理结构并对其进行扩展；在分析基于代理的车间调度系统模型的基础上对车间调度系统的代理协商过程、协商协议及协商步骤进行了探讨、研究和改进。     

An investigation of labor assignment rules in a dual-constrained job shop

One of the management decisions required to operate a dual-constrained job shop is the labor assignment rule. This study examines the effects of various labor assignment rules on the shop's performance. Eleven different labor assignment rules are simulated. A longest-queue rule and the traditional counterparts of the first-in-system, first-served, shortest operation time, job due date, critical ratio and shortest processing time dispatching rules are used to determine to which work center available workers should be transferred. Also tested are five new labor assignment rules that use an average of the priority values of all jobs in queue at a particular work center to determine whether that work center should receive the available worker.A SIMSCRIPT simulation program that models nine work centers provided the mechanism by which these rules were tested. Five dispatching rules, the counterparts of the five “traditional counterpart” labor assignment rules mentioned earlier, provided different shop environments. Also, the level of staffing of the work centers was altered to provide additional ship environments. Staffing levels of 50% and 67% were employed.The results show that none of the eleven labor assignment rules had a significant impact on shop performance. This is an important result because it implies that a manager can make the labor assignment decision based on other criteria such as ease or cost of application of the rules. These results were relatively insensitive to the shop environment, as represented by the dispatching rule and the staffing level.     

Preferences,Comparative Advantage,and Compensating Wage Differentials for Job Routinization*

I attempt to explain why there is not much evidence on compensating wage differentials for job disamenities. I focus on the match between workers’ preferences for routine jobs and the variability in tasks associated with the job. Using data from the Wisconsin Longitudinal Study, I find that mismatched workers earn lower wages and that both male and female workers in routinized jobs earn, on average, 5.5% and 7% less than their counterparts in non‐routinized jobs. However, once preferences and mismatch are accounted for, this difference decreases to 2% for men, and 4% for women, not statistically significant in both cases.     

Job shop scheduling with modified due dates

This paper introduces a new dispatching rule to job shop scheduling, extending earlier results to a multi-machine environment. This new rule, which uses modified due dates is compared to other popular dispatching methods over a range of due date tightnesses at two utilization levels. The results for mean tardiness indicate that the modified operation due date (MOD) rule compares very favorably with other prominent dispatching methods.The modified due date is a job's original due date or its early finish time, whichever is larger. For an individual operation, it is the operation's original due date or the operation's early finish time, whichever is larger. A comparison of the job-based version with the operation-based version indicated that the operation-based version tended to be more effective at meeting due dates.The main performance measure was mean job tardiness, although the proportion of tardy jobs was also reported, and the two measures together imply the conditional mean tardiness. The MOD rule was compared to several well-known tardiness-oriented priority rules, such as minimum slack-per-operation (S/OPN), smallest critical ratio (SCR), and COVERT. The MOD rule tended to achieve lower levels of mean tardiness than the other rules, except under conditions where due dates were quite loose. In this situation, very little tardiness occurs for any of the rules. The MOD rule appeared to be more robust than the other rules to changes in the tightness of due dates, and similar results occurred at both high and low utilizations.     

Algorithms as work designers: How algorithmic management influences the design of jobs

We review the literature on algorithmic management (AM) to bridge the gap between this emerging research area and the well-established theory and research on work design. First, we identify six management functions that algorithms are currently able to perform: monitoring, goal setting, performance management, scheduling, compensation, and job termination. Second, we show how each AM function affects key job resources (e.g., job autonomy, job complexity) and key job demands (e.g., workload, physical demands); with each of these resources and demands being important drivers of worker motivation and their well-being. Third, rejecting a deterministic perspective and drawing on sociotechnical systems theory, we outline key categories of variables that moderate the link between AM on work design, namely transparency, fairness and human influence (e.g., whether workers can control the system). We summarize our review in the form of a model to help guide research on AM, and to support practitioners and designers in the creation and maintenance of meaningful jobs in the era of algorithms.     

Small,Local and Cheap? Walkable and Car-oriented Retail in Competition

Abstract

I develop a model of competition between walkable shops, and other shops whose customers drive (car-oriented shops). Walkable shops operate in monopolistic competition within a local area, or neighbourhood. A small cost advantage for car-oriented shops can turn into a larger price advantage. High prices in walkable shops effect a regressive transfer from poorer to richer consumers, since the poorer are less likely to have cars. Internalizing environmental and social costs of urban automobile use could reduce prices and increase capacity utilization in walkable shops in more densely populated local areas. Many common combinations of planning and pricing tools fail to internalize important costs, and may actually subsidize driving to shop, but a combination of planning and the pricing (through taxation) of retail parking could effectively internalize the relevant costs.