Walid Nasr - Academia.edu (original) (raw)

Papers by Walid Nasr

Research paper thumbnail of Effect of deterioration on the instantaneous replenishment model with imperfect quality items

Applied Mathematical Modelling, 2014

ABSTRACT The assumptions required to justify the use of the economic order quantity model (EOQ) a... more ABSTRACT The assumptions required to justify the use of the economic order quantity model (EOQ) are rarely met. To provide mathematical models that more closely represent real-life situations, these assumptions must be relaxed. Among these assumptions are, first, items stocked are of perfect quality, and second, they preserve their characteristics during their stay in inventory. This paper considers a modified EOQ-type inventory model for a deteriorating item with unreliable supply. That is, a percentage of the on-hand inventory is wasted due to deterioration. Moreover, orders may contain a random proportion of defective items, which follow a known distribution. As soon as an order is received, a retailer conducts a screening process to identify imperfect quality items, which are salvaged as a single batch at the end of the screening process. First, a mathematical model is developed, assuming that no shortages are allowed. For that, it is assumed that the inventory level when placing an order is just enough to cover the demand during the screening period. The concavity of the profit function is established and sensitivity analysis is provided to analyze the impact of changing various model parameters on the optimal order quantity and profit. Then, the assumption of no shortages is relaxed, and a model is developed to incorporate backorders. We analyze the model with backorders numerically and provide managerial insights.

Research paper thumbnail of Integrating the economic production model with deteriorating raw material over multi-production cycles

International Journal of Production Research, 2014

ABSTRACT This paper considers an Economic Production Quantity (EPQ) model with deteriorating raw ... more ABSTRACT This paper considers an Economic Production Quantity (EPQ) model with deteriorating raw material and investigates the impact of deterioration on the production process. The EPQ base case with no deterioration is presented where raw material is ordered for multiple production cycles. We present the differential equations to calculate the on-hand inventory of raw material and present closed-forms expressions for the required order of raw material to result in a desired amount of effective raw material per order cycle. Closed-form expression for the total profit per unit time is obtained and we solve for the optimal production quantity of finished product per production cycle and the order quantity of raw material. We present numerical examples where we compare our model to a system which ignores the impact of deterioration and results in shorter production cycles due to an insufficient amount of effective raw material.

Research paper thumbnail of EOQ with a correlated binomial supply

International Journal of Production Economics, 2013

ABSTRACT This paper considers a variant of the economic order quantity (EOQ) model under random s... more ABSTRACT This paper considers a variant of the economic order quantity (EOQ) model under random supply. A binomial supply model is adopted where every item received is of imperfect quality with the same probability. We study the realistic case where the qualities of items in an order are correlated and draw useful insights, the most interesting of which is that correlation decreases the order size. Several practical correlation patterns are investigated and discussed. We also show that several commonly used models in the literature can be represented by an equivalent correlated binomial supply model.

Research paper thumbnail of Fitting the Ph t / M t / s / c Time-Dependent Departure Process for Use in Tandem Queueing Networks

INFORMS Journal on Computing, 2013

ABSTRACT This paper considers time-dependent Ph-t/M-t/s/c queueing nodes and small tandem network... more ABSTRACT This paper considers time-dependent Ph-t/M-t/s/c queueing nodes and small tandem networks of such nodes. We examine characteristics of the departure processes from a multiserver queueing node; in particular, we focus on solving for the first two time-dependent moments of the departure-count process. A finite set of partial moment differential equations is developed to numerically solve for the departure-count moments over specified intervals of time [t(i), t(i) + tau(i)). We also present a distribution fitting algorithm to match these key characteristics with a (Ph-t) over tilde process serving as the approximate departure process. A distribution fitting algorithm is presented for time-dependent point processes where a two-level balanced mixture of Erlang distribution is used to serve as the approximating process. We then use the (Ph-t) over tilde approximating departure process as the approximate composite arrival process to downstream node(s) in a network of tandem queues.

Research paper thumbnail of Transshipment and safety stock under stochastic supply interruption in a production system

Computers & Industrial Engineering, 2012

We consider a two-echelon system with one source supplying two locations with the same product. T... more We consider a two-echelon system with one source supplying two locations with the same product. The random occurrence of interruptions at the source where downtime is also stochastic can result in stockouts at the two receiving locations. Our model studies the benefit of allowing each location to carry a safety stock where holding costs can be different at each location. The objective is to reduce overall cost at both locations. In some cases it is optimal to allow for a transshipment of inventory from the safety stock of one location to the other. We jointly solve for the optimal safety stock at each location and the optimal amount to be transshipped from a location to the other. We show that by conditioning on the transshipment direction the total cost becomes convex as a function of the safety stock levels at the receiving locations and the amount to be transshipped from a location to the other. Numerical examples are presented for different system cost parameters and probability distributions.

Research paper thumbnail of An instantaneous replenishment model under the effect of a sampling policy for defective items

Applied Mathematical Modelling, 2013

ABSTRACT We propose to study a EOQ-type inventory model with unreliable supply, with each order c... more ABSTRACT We propose to study a EOQ-type inventory model with unreliable supply, with each order containing a random proportion of defective items. Every time an order is received, an acceptance sampling plan is applied to the lot, according to which only a sample is inspected instead of the whole lot. If the sample conforms to the standards, i.e. if the number of imperfect items is below an “acceptance number”, no further screening is performed. Otherwise, the lot is subject to 100% screening. We formulate an integer non-linear mathematical program that integrates inventory and quality decisions into a unified profit model, to jointly determine the optimal lot size and optimal sampling plan, characterized by a sample size, and an acceptance number. The optimal decisions are determined in a way to achieve a certain average outgoing quality limit (AOQL), which is the highest proportion of defective items in the outgoing material sold to customers. We provide a counter-example demonstrating that the expected profit function, objective of the mathematical program, is not jointly concave in the lot and sample size. However, we show that for a given sampling plan, the expected profit function is concave in the lot size. A solution procedure is presented to compute the optimal solution. Numerical analysis is provided to gain managerial insights by analyzing the impact of changing various model parameters on the optimal solution. We also show numerically that the optimal profit determined using this model is significantly higher when compared to the optimal profit obtained using Salameh and Jaber (2000)’s [1] model, indicating much higher profits when acceptance sampling is used.

Research paper thumbnail of Effect of deterioration on the instantaneous replenishment model with imperfect quality items

Applied Mathematical Modelling, 2014

ABSTRACT The assumptions required to justify the use of the economic order quantity model (EOQ) a... more ABSTRACT The assumptions required to justify the use of the economic order quantity model (EOQ) are rarely met. To provide mathematical models that more closely represent real-life situations, these assumptions must be relaxed. Among these assumptions are, first, items stocked are of perfect quality, and second, they preserve their characteristics during their stay in inventory. This paper considers a modified EOQ-type inventory model for a deteriorating item with unreliable supply. That is, a percentage of the on-hand inventory is wasted due to deterioration. Moreover, orders may contain a random proportion of defective items, which follow a known distribution. As soon as an order is received, a retailer conducts a screening process to identify imperfect quality items, which are salvaged as a single batch at the end of the screening process. First, a mathematical model is developed, assuming that no shortages are allowed. For that, it is assumed that the inventory level when placing an order is just enough to cover the demand during the screening period. The concavity of the profit function is established and sensitivity analysis is provided to analyze the impact of changing various model parameters on the optimal order quantity and profit. Then, the assumption of no shortages is relaxed, and a model is developed to incorporate backorders. We analyze the model with backorders numerically and provide managerial insights.

Research paper thumbnail of Integrating the economic production model with deteriorating raw material over multi-production cycles

International Journal of Production Research, 2014

ABSTRACT This paper considers an Economic Production Quantity (EPQ) model with deteriorating raw ... more ABSTRACT This paper considers an Economic Production Quantity (EPQ) model with deteriorating raw material and investigates the impact of deterioration on the production process. The EPQ base case with no deterioration is presented where raw material is ordered for multiple production cycles. We present the differential equations to calculate the on-hand inventory of raw material and present closed-forms expressions for the required order of raw material to result in a desired amount of effective raw material per order cycle. Closed-form expression for the total profit per unit time is obtained and we solve for the optimal production quantity of finished product per production cycle and the order quantity of raw material. We present numerical examples where we compare our model to a system which ignores the impact of deterioration and results in shorter production cycles due to an insufficient amount of effective raw material.

Research paper thumbnail of EOQ with a correlated binomial supply

International Journal of Production Economics, 2013

ABSTRACT This paper considers a variant of the economic order quantity (EOQ) model under random s... more ABSTRACT This paper considers a variant of the economic order quantity (EOQ) model under random supply. A binomial supply model is adopted where every item received is of imperfect quality with the same probability. We study the realistic case where the qualities of items in an order are correlated and draw useful insights, the most interesting of which is that correlation decreases the order size. Several practical correlation patterns are investigated and discussed. We also show that several commonly used models in the literature can be represented by an equivalent correlated binomial supply model.

Research paper thumbnail of Fitting the Ph t / M t / s / c Time-Dependent Departure Process for Use in Tandem Queueing Networks

INFORMS Journal on Computing, 2013

ABSTRACT This paper considers time-dependent Ph-t/M-t/s/c queueing nodes and small tandem network... more ABSTRACT This paper considers time-dependent Ph-t/M-t/s/c queueing nodes and small tandem networks of such nodes. We examine characteristics of the departure processes from a multiserver queueing node; in particular, we focus on solving for the first two time-dependent moments of the departure-count process. A finite set of partial moment differential equations is developed to numerically solve for the departure-count moments over specified intervals of time [t(i), t(i) + tau(i)). We also present a distribution fitting algorithm to match these key characteristics with a (Ph-t) over tilde process serving as the approximate departure process. A distribution fitting algorithm is presented for time-dependent point processes where a two-level balanced mixture of Erlang distribution is used to serve as the approximating process. We then use the (Ph-t) over tilde approximating departure process as the approximate composite arrival process to downstream node(s) in a network of tandem queues.

Research paper thumbnail of Transshipment and safety stock under stochastic supply interruption in a production system

Computers & Industrial Engineering, 2012

We consider a two-echelon system with one source supplying two locations with the same product. T... more We consider a two-echelon system with one source supplying two locations with the same product. The random occurrence of interruptions at the source where downtime is also stochastic can result in stockouts at the two receiving locations. Our model studies the benefit of allowing each location to carry a safety stock where holding costs can be different at each location. The objective is to reduce overall cost at both locations. In some cases it is optimal to allow for a transshipment of inventory from the safety stock of one location to the other. We jointly solve for the optimal safety stock at each location and the optimal amount to be transshipped from a location to the other. We show that by conditioning on the transshipment direction the total cost becomes convex as a function of the safety stock levels at the receiving locations and the amount to be transshipped from a location to the other. Numerical examples are presented for different system cost parameters and probability distributions.

Research paper thumbnail of An instantaneous replenishment model under the effect of a sampling policy for defective items

Applied Mathematical Modelling, 2013

ABSTRACT We propose to study a EOQ-type inventory model with unreliable supply, with each order c... more ABSTRACT We propose to study a EOQ-type inventory model with unreliable supply, with each order containing a random proportion of defective items. Every time an order is received, an acceptance sampling plan is applied to the lot, according to which only a sample is inspected instead of the whole lot. If the sample conforms to the standards, i.e. if the number of imperfect items is below an “acceptance number”, no further screening is performed. Otherwise, the lot is subject to 100% screening. We formulate an integer non-linear mathematical program that integrates inventory and quality decisions into a unified profit model, to jointly determine the optimal lot size and optimal sampling plan, characterized by a sample size, and an acceptance number. The optimal decisions are determined in a way to achieve a certain average outgoing quality limit (AOQL), which is the highest proportion of defective items in the outgoing material sold to customers. We provide a counter-example demonstrating that the expected profit function, objective of the mathematical program, is not jointly concave in the lot and sample size. However, we show that for a given sampling plan, the expected profit function is concave in the lot size. A solution procedure is presented to compute the optimal solution. Numerical analysis is provided to gain managerial insights by analyzing the impact of changing various model parameters on the optimal solution. We also show numerically that the optimal profit determined using this model is significantly higher when compared to the optimal profit obtained using Salameh and Jaber (2000)’s [1] model, indicating much higher profits when acceptance sampling is used.