Kum Khiong Yang

Scheduling a project to maximize its net present value: An integer programming approach

We describe an integer programming algorithm for determining scheduled start and finish times for the activities of a project subject to resource limitations during each period of the schedule duration. The objective is to maximize the net present value of the project to the firm. A depth-first branch and bound solution procedure searches over the feasible set of finish or completion times for each of the activities of the project. Fathoming criteria based upon the concept of a network cut originally developed to solve the duration minimization version of this problem are extended in this paper to solve the net present value problem. These fathoming decision rules prevent many potentially inferior solutions from being explicitly evaluated. Computational experience reported demonstrates the efficacy of the approach.

A comparison of stochastic scheduling rules for maximizing project net present value

This paper examines the use of stochastic scheduling rules for maximizing the net present value of a project. A comprehensive set of 1440 test problems, representing five different project characteristics, is constructed. These test problems are used to evaluate the performance of nine stochastic scheduling rules. A simulated annealing scheduling procedure is shown to perform best, generating the highest net present values for most of the test problems. In certain environments, two previously examined rules, the Rank Positional Weight and a Discounted Cumulative Cash Flow Weight rule, also perform well with high net present values.

A study on manufacturing resource planning (MRP II) practices in Singapore

In response to the increasing regional and international competition, many manufacturing companies in Singapore have adopted MRP II systems to improve on their business operations. Despite its growing popularity, very little work has been done to monitor how companies are actually using MRP II. This paper presents the largest survey study ever undertaken on the state-of-the-art application of MRP II in Singapore. It describes the major survey findings on the practices, costs and benefits obtained, and implementation process in MRP II companies. This study provides useful information to managers, current users, potential users, vendors and promoters of MRP II. The study also serves as a basis for future research. International readers are presented with valuable insights into how manufacturing companies in a successful newly industrializing nation (NIE) like Singapore are exploiting a powerful technology like MRP II to manage their operations. A comparison of the survey findings with that of a similar study in the United States reveals an interesting difference.

Comparison of make-to-order job shops with different machine layouts and production control systems

SUMMARY This research project examines the performance of job shops with different machine layouts and production control systems. Job shops examined in this study operate in a make-to-order environment. Results indicate that the material requirements planning order-release-and-due-date-assignment procedure always performs better than that of the period batch control. Cellular layout, however, performs better at a high setup-to-processing-time ratio, while the process layout performs better at a low setup-to-processing-time ratio

A comparison of cross-training policies in different job shops

This research compares a set of cross-training policies represented by different numbers of cross-trained workers, additional skills per cross-trained worker, and additional machines. The policies are evaluated in job shops, represented by different efficiency losses, labour utilization, variability in processing times, and worker absenteeism. Our results show that adding one machine in each department and cross-training one or two workers from each department with one additional skill is generally sufficient to realize most of the benefits of cross-training. Cross-training is thus beneficial in most job shops, unless the cost of the minimal training and spare machines is high. Our results also show that the value of cross-training and adding machines depends very much on the environment, and it is better to spread cross-training over more workers than to train a few workers with more skills.

A comparison of job shop dispatching rules using a total cost criterion

Abstract Recent research shows existing dispatching rules, proposed for minimizing the total cost, do not perform better than the Critical Ratio rule. New dispatching rules are, therefore, proposed in this paper and are shown to perform significantly better than existing dispatching rules. Under certain conditions of high tardiness penalty rate, high machine utilization, and tight due date, the new dispatching rules result in as much saving as 35% of the total cost incurred by the Critical Ratio rule.

An evaluation of worker cross training and flexible workdays in job shops

Capacity flexibility is becoming increasingly important as a means for reducing inventory while maintaining customer service levels. We examine two means to increase capacity flexibility. In particular, we examine an environment where both cross training and flexible workdays are available to respond to workload variability. Flexible workdays are under consideration in the US Legislature. This proposed legislation provides the opportunity for workers to exchange overtime for time off. From a managerial perspective, flexible workdays allow management to shift capacity from periods of light load to periods of heavy load. We simulate the operation of a job shop with both cross training and flexible workdays. Our results indicate that cross training is a far more effective tool for improving performance as compared to flexible workdays. Flexible workdays can be valuable particularly in volatile conditions. However, our results indicate that the degree of cross training is a critical consideration in determining the impact of flexible workdays.

A comparison of dispatching rules for executing a resource-constrained project with estimated activity durations

This research examines the performance of 13 dispatching rules for executing a resource-constrained project whose estimated activity durations may differ from the actual activity durations. The dispatching rules are tested in environments characterized by three factors, namely, the order strength of the precedence relationship, the level of resource availability and the level of estimation errors in the activity durations. The results show that project environment affects only the performance differences but not the grouping of the better dispatching rules. The greatest number of successors, rank positional weight, greatest cumulative resource requirement and minimum activity slack dispatching rules consistently perform better than the other dispatching rules, unaffected by the accuracy of the estimated activity durations. This finding validates the results of many past studies in the deterministic project environment for choosing the right dispatching rule for both projects with and without well-estimated activity durations.

Exploratory analysis of flexible workday policies in a job shop

The degree to which simple policies for varying the length of each workday can affect job shop performance was investigated. The performance of a job shop operating on a fixed 8-hour day schedule was compared with a job shop that had some flexibility to vary an employees workday by compensating hours in excess of an 8-hour day with an equivalent amount of time off. Thus, each flexible workday policy maintains an average workday of 8 hours/day for each employee. We simulated a hypothetical job shop to assess the effects of various environmental factors and workday policies on measures of interest to management and labour. We found significant improvement in job flow time with overtime (and time off) as little as an average of 24 min for every 8 hours of work.

An Evaluation of Flexible Workday Policies in Job Shops

Job shops have long faced pressures for improvement in a challenging and volatile environment. Todays 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.