Robert C. Carlson

Interaction between frequency of rescheduling and the role of safety stock in material requirements planning systems

We describe how the frequency of rescheduling permitted in Material Requirements Planning (MBP) systems influences the impact of safety stock on system costs, and its effectiveness in maintaining desired levels of customer service. The results are obtained from approximate analytical models and simulation studies of a single product with stochastic demand and a two-level product structure. The results indicate that in some cases it may be more economical to reschedule infrequently and use safety stock as protection against demand variations. They also indicate that the effect of changing safety stock levels is much more predictable when rescheduling is infrequent, and that increasing safety stock may actually result in degraded performance when rescheduling is frequent.

The Simple Plant-Location Problem under Uncertainty

There is a class of simple plant-location problems under uncertainty that can be decomposed into two simpler problems that can be solved sequentially with surprising ease. The salient characteristic of this class of problems is that for any demand generating region, one plant dominates all others as a supply source. We develop and discuss the conditions required for this dominance. The risk-averse nature of the firm is incorporated into this model with a mean-variance formulation of the objective function. The general formulation is particularized to four paradigms of firms characterized by a range of behavioral and market assumptions.

Designing a Production Line to Maximize Profit

Abstract The relationship between profitability and efficiency in a production line is discussed at length for the continuous production model. We prove that a solution which maximizes efficiency will not necessarily maximize profit. Several useful relationships between profitability and efficiency are developed which can greatly enhance computational efficiency. A solution procedure is developed which will determine the optimal number of work stations by maximizing an assumed profit function. An example is provided to illustrate these relationships and the entire solution procedure.

An analysis of scheduling policies in multiechelon production systems

Abstract Results of a simulation study of the economics of frequency of rescheduling Material Requirements Planning (MRP) systems are presented for a single-product, two-stage system in which demand is uncertain. The results indicate that for systems with moderate demand uncertainty, frequent rescheduling to maintain customer service may be uneconomical when compared with the alternative of more stable schedules in conjunction with safety stock. This result arises primarily because the cost of “emergency” production setups which occur when rescheduling is frequent exceeds the cost of safety stock required to “protect” stable schedules.

Dynamic Lot-Sizing When Demand Timing Is Uncertain

We characterize optimal solutions for the dynamic lot-sizing problem when demand quantities are known, but their timing is uncertain. The characterization states that for some possible history of demands following a period of production, there is a period with zero inventory before another period with production. Using this characterization, we outline a dynamic program for the solution of these problems. Some results from sample problems illustrate the variety of optimal solutions that can occur.

Sequencing of Endocrine Therapies in Breast Cancer – Integration of Recent Data

A wide range of endocrine therapies has demonstrated activity in the treatment of hormone receptor-positive metastatic breast cancer and sequential tumor responses to sequential hormonal therapies are common. However, the optimal sequence of the hormonal therapies has not yet been determined. The selection of endocrine therapies in women with hormone receptor-positive breast cancer is strongly influenced by the menopausal status of the patient. For premenopausal women, tamoxifen alone or combined with ovarian suppression using a luteinizing hormone-releasing hormone (LHRH) agonist – such as goserelin or leuprolide – is an appropriate first-line hormonal therapy. Ovarian ablation or megestrol acetate is an appropriate second-line hormonal therapy for premenopausal women treated with tamoxifen as first-line therapy, or ovarian ablation plus an aromatase inhibitor (AI) or megestrol acetate for women treated with first-line tamoxifen plus an LHRH agonist. For postmenopausal women, the non-steroidal AIs anastrozole and letrozole now represent the preferred first-line hormonal treatment for metastatic breast cancer, based upon both efficacy and toxicity considerations. For second-line therapy in postmenopausal women, a number of options now exist, including tamoxifen, the steroidal AI exemestane, and the new agent fulvestrant. Fulvestrant, a novel estrogen receptor (ER) antagonist that downregulates the ER and has no known agonist effects, has been demonstrated to be at least as effective as anastrozole in postmenopausal women whose tumors progress on tamoxifen. The establishment of the optimal sequence of the endocrine therapies should offer significant benefits to women with hormone-sensitive metastatic breast cancer.

Dynamic order promising: real-time ATP

In order to satisfy customer demands despite unique specifications or schedule constraints, manufacturers have realised the critical need for real-time Available-To-Promise (ATP). In this paper, we present a model for real-time order promising in a mixed make-to-order, make-to-stock manufacturing environment. Each fulfilment source is considered as a separate module. Consistent with the real-time nature of the problem, this model considers a snapshot view of the enterprise at the moment the customer order enters the system. Relevant values from potential fulfilment sources are passed to the ATP optimisation engine. Following an instantaneous decision to accept or reject the order, the newly pegged resources are updated in the system. The flexibility in this modular structure allows the model to adapt to the most fragmented IT system or leverage the benefits of a highly integrated ERP system. We found that order acceptance levels and costs are most sensitive to capacity utilisation. Other factors that showed significant effects in our real-time ATP environment were demand variability, number or orders per day and the magnitude of these orders.

The Simultaneous Determination of Plant and Leased Warehouse Capacities for a Firm Facing Uncertain Demand in Several Regions

Abstract For each of several regions a firm has forecasted a probability distribution of future annual demand, and the plan is to build a single production facility to serve this demand. Each region is served by a simple leased warehouse with a lease cost which is a linear function of capacity. Any regional demand exceeding warehouse capacity is lost. The cost of building the plant is a nonlinear function of its capacity, and the unit cost of producing the product and supplying it to a region is constant. We develop an efficient algorithm based on the Kuhn-Tucker conditions for simultaneously determining the plant and warehouse capacities which will maximize the firms expected profit. The paper includes a numerical example demonstrating the use of this algorithm.

Anatomy of a systems failure: Dial-a-ride in Santa Clara County, California

AbstractLate in 1974 and early in 1975 the Santa Clara County Transit District initiated, operated, and then discontinued a demand responsive dial-a-ride system within a