No description available.

No description available.

No description available.

No description available.

A computer simulation experiment was conducted to evaluate and compare five individual forecasting models across nine different demand patterns. The models were based on the Medical Materiel Management System used by the US Air Force hospitals. Results indicated the best model varied depending on the demand pattern, the safety stock level, the noise level of the demand pattern, and the measure of forecast error. Across all demand patterns, exponential smoothing and 12-month moving average were best for the short term forecast used by the system, regardless of noise level in the demand patterns. Analysis of models within a single demand pattern showed, in most cases, several models as ranking equally well. When overall system requirements were considered, the exponential smoothing method was by far the best choice.

No description available.

This work considers inventory systems with two demand classes, where advance demand information is available. Three related scenarios are presented: three-stage production-inventory systems are studied in first two, whereas pure inventory systems are studied in the last scenario. In the first scenario, continuous review production-inventory systems are considered, where only one class provides advance demand information and early demand fulfillment is permitted. A new approach for production replenishment and order fulfillment in such systems is proposed, which combines the benefits of early fulfillment with Kanban-based pull systems. Simulation is used to compare the performance of the resulting policy with two other policies for a variety of scenarios (depending on the arrival rates, system utilizations, cost structures, arrival ratio, priority levels and amount of the advance demand information). A simulation-based lower bound on the optimal cost is established for some specific scenarios. The proposed policy outperforms the existing policies in every setting considered. Also, the proposed policy has added advantage of both retaining the benefit at high system utilizations and increasing the benefit up to the maximum level of advance demand information provided. A small fraction of customers providing advance demand information with early fulfillment acceptable is shown to have higher benefit than all customers providing same advance demand information with no early fulfillment. In second scenario, both classes provide advance demand information in production-inventory systems, though only one class accepts early fulfillment. Different levels of system utilization, arrival ratio and backorder cost are considered in the simulation experiments to show the superiority of early fulfillment. Also, experiments suggest that lowering the expected supply lead time may be more beneficial than increasing the demand lead time by the same amount for production-inventory systems with utilization dependent supply lead times. In third scenario, pure inventory systems are considered, where the demand classes provide different amount of advance demand information, and only one class accepts early fulfillment. The structure of an optimal policy is analytically characterized for periodic review systems under some specific conditions. / Ph. D.

In this research, we consider the spares provisioning problem, where a finite population of homogeneous machines are being deployed to meet a constant demand. While a machine is operating, it could become inoperable due to the failure of a critical built-in part in the machine. Before repairs on the machine can be initiated, however, a replacement part must be obtained. If a replacement part is available from stock, the machine is immediately transferred to the repair subsystem, in which one or more repair stations operate in parallel. If the replacement part is not in stock, then the machine waits in the ordering subsystem for the arrival of a new part. Once a machine is repaired, it is immediately deployed to meet demand if needed, else it joins a queue of standby machines. The spare machines have zero probability of failure and, if available, a spare replaces a deployed machine immediately upon the latter's failure. The machine operating time, repair time, and ordering time of the parts are assumed to be exponentially distributed. The ordering subsystem for the parts brings a new aspect to the spares provisioning problem, and dramatically increases its difficulty. This is because the queuing network model which describes the system is a non-product-form network in the case of finite nonzero stocking policy, and specification of closed-form solutions is highly unlikely for such networks. In this dissertation, we present efficient algorithms through which the optimal number of machines, repair stations and stocking level of the parts that minimize total operations costs subject to a service-level constraint can be obtained. The algorithms, which based on Little's result from queuing theory and some approximate models used for bounding, have proven to be extremely efficient in terms of computer storage and execution time, even for large problems. / Ph. D.

A specialized lot size model for procurement of direct material items is applied in a specific manufacturing firm. Simulation studies are utilized for determining the effectiveness of the lot size model for use in a multiproduct MRP production system having a specific product structure. Measures of effectiveness with respect to cost categories including shortage frequency, material costs and holding costs will be obtained based on a range of demand patterns and levels. These studies focus on five end items representative of the population of brands manufactured at the company under consideration. The results of the study provide a methodological basis for system wide implementation of model based material ordering policy. / M.S.

Currently many American industrial firms are considering the adoption of Just-In-Time (JIT) as an inventory control/material flow system. A JIT system can have several configurations. Examples exist of JIT being applied system-wide between all the echelons which make up a channel. There are also examples of firms adopting JIT only on the materials management side, or alternatively, only on the physical distribution side of a particular focal organization (echelon) within a channel. The reality of uncertainty in the channel's operation and environment suggests that alternative inventory systems (such as JIT) must be evaluated under a range of internal operational uncertainty and external demand uncertainty conditions. This study offers a rational methodology to anticipate the performance impact of alternative system designs under realistic operating conditions. The G.P.M. simulation model was used to represent the operation of a channel under sixteen treatment conditions. The research design was full factorial with two levels each of four factors (physical distribution JIT, materials management JIT, materials management operational uncertainty, and demand uncertainty). The response variables which were used as indicators of channel performance included: profitability, order cycle time, standard deviation of order cycle time, and percent customer orders filled. Eleven research hypotheses explored the relationship between JIT configuration and uncertainty, both in terms of profitability and physical distribution service level maintained. The major conclusions of the research are: 1. JIT is not the unidimensional system often depicted in descriptive studies. JIT effects tend to be complex, interactive, and level dependent. It is particularly difficult to predict the effect of JIT on one echelon, or on subsystems within echelons. 2. Rather than the inherent positive effects often attributed to JIT, results indicate negative effects for both profit and service under a range of uncertainty conditions. 3. Results support the common criticism of JIT that its performance is sensitive to uncertainty, particularly demand uncertainty. The performance of non-JIT systems were also shown to have similar sensitivity to uncertainty. 4. Most synergistic interactions between factors were not significant, but the statistical procedure for means comparison was acknowledged to be conservative. 5. Results also indicated that JIT systems may make the job of maintaining high customer service levels more difficult. / Ph. D.