technical note note on myopic heuristics for the random yield problem bollapragada and morton 1999 present several well performing heuristics for solving the periodic inventory problem with random yield and demand their approach is essentially based on a transformation of the single period problem into a standard newsvendor problem with deterministic yield and random demand which however is supply dependent in our note we show that their evaluation of the respective optimality condition is not correct this explains the steady deterioration of their myopic heuristics for parameter constellations that correspond to increasing service levels some computational investigations reveal that the performance of the heuristics can become quite poor if service levels are high and exceed those values for which results are reported in the original study nonetheless up to now these heuristics are still the best ones available for solving the joint random yield problem

dynamic replenishment from two sources with different yields costs and leadtimes abstract we study a single item periodic review inventory system with two sources of supply in house production and outsourcing the two sources differ in yields costs and leadtimes demands in consecutive periods are independent random variables and stockout is fully backlogged our problem is to characterize the optimal dynamic policy that simultaneously determines the in house production quantity and the outsourcing quantity for each period to minimize the total discounted cost we show that the optimal policy for in house production can be characterized by a threshold and the production quantity is decreasing in the starting inventory the optimal outsourcing policy however is more complex and a threshold type policy is only optimal under a more restrictive assumption about the yields comparative statics about the effects of yield uncertainty outsourcing on the optimal policy and cost performance are discussed

the value of real time yield information in multi stage inventory systems exact and heuristic approaches abstract we consider a random yield inventory system where a company has access to real time information about the actual yield realizations to contribute to a better understanding of the value of this information we develop a mathematical model of the inventory system and derive structural properties we build on these properties to develop an optimal solution approach that can be used to solve small to medium sized problems to solve large problems we develop two heuristics we conduct numerical experiments to test the performances of our approaches and to identify conditions under which real time yield information is particularly beneficial our research provides the approaches that are necessary to implement inventory control policies that utilize real time yield information the results can also be used to estimate the cost savings that can be achieved by using real time yield information the cost savings can then be compared against the required investments to decide if such an investment is profitable

time to build option value and investment decisions a comment abstract we correct the analysis of the model of time to build in majd and pindyck 1987 journal of financial economics 18 7 27 for the omission of an essential optimality condition our analysis reveals an additional insight long times to build reduce the effects of increased project value volatility i e higher investment thresholds in comparison to standard real option models of investment under uncertainty where investment is instantaneous thus a naive npv rule can sometimes be an appropriate initial guide to investment

a periodic review lot sizing problem with random yields disruptions and inventory capacity this paper examines a periodic reviewed lot sizing problem with random yields disruptions and limited inventory capacity to characterise the continuous production an additive random yield model is considered rather than a multiplicative one disruptions cause breakdowns to production inventory capacity is included since the production has to be shut down when the inventory buffer is full both disruptions and shutdowns lead to a start up cost and a stochastic lead time to recover the production these compound factors of uncertainty are encountered in practical planning decisions in process industries we review the existing random yield models which are then compared with the additive model with a linear production cost the additive model has an order up to policy to be optimal disruptions deteriorate the expected actual production quantity and the fill rate dramatically even though the optimal order up to level increases compared with the cases of no disruption considering inventory capacity makes the problem to be a non convex dynamic programming problem numerical analysis shows that the performance is dramatically deteriorated when the inventory capacity is rather tight which indicates the importance of selecting a proper inventory capacity to reduce the negative impacts and avoid redundant investment on capacity moreover the start up cost plays an important role in determining the level of inventory capacity

role of risk aversion in price postponement under supply random yield price postponement is an effective mechanism to hedge against the adverse effect of supply random yield however its effectiveness and the resulting production decisions have not been studied for

do business cycle conditions at the time of labor market entry affect future employment prospects abstract labor market conditions at the time and place of potential entry into the labor market are shown to have a substantial and persistent effect on adult employment prospects individuals who face particularly depressed local labor markets when they graduate from secondary education are other things equal subject to relatively high rates of nonemployment during their whole prime age work career building on a unique combination of micro and macro data from norway we show that these effects are robust with respect to model specification and conditioning variables and that they are not limited to individuals with a particularly disadvantaged background

estimates of the economic returns to schooling from a new sample of twins this paper uses a new survey to contrast the wages of genetically identical twins with different schooling levels multiple measurements of schooling levels were also collected to assess the effect of reporting error on the estimated economic returns to schooling the data indicate that omitted ability variables do not bias the estimated return to schooling upward but that measurement error does bias it downward adjustment for measurement error indicates that an additional year of schooling increases wages by 12 to 16 percent a higher estimate of the economic returns to schooling than has been previously found copyright 1994 by american economic association

a bargaining model for a first time interaction under asymmetric beliefs of supply reliability we consider the case of a first time interaction between a buyer and a supplier who is unreliable in delivery the supplier declares her estimate of the ability to meet the order obligations but the buyer may have a different estimate which may be higher or lower than the suppliers estimate we derive the nash bargaining solution and discuss the role of using a down payment or nondelivery penalty in the contract for the case of buyer overtrust the down payment contract maximizes channel profits when the suppliers estimate is public information if the suppliers estimate is private information a nonsymmetric contract is shown to be efficient and incentive compatible for the case of buyer undertrust the contract structure is quite different as both players choose not to include down payments in the contract when delivery estimates are public information a nondelivery penalty contract is able to maximize channel profits if the buyer uses the suppliers estimate in making the ordering decision if estimates are private information channel profits are maximized only if the true estimates of both players are not far part we also discuss the effect of different risk profiles on the nature of the bargaining solution in three extensions of the model we consider the following variants of the basic problem first we analyze the effect of early versus late negotiation on the bargaining solution then we study the case of endogenous supply reliability and finally for the case of repeated interactions we discuss the impact of updating delivery estimates on the order quantity and negotiated prices of future orders

using unemployment rates as instruments to estimate returns to schooling i use state unemployment rates during a person s teenage years to estimate the returns to schooling a higher unemployment rate reduces the opportunity costs of attending school using the same 1980 census data set that angrist and krueger 1991 use i also estimate returns to schooling with a modified version of their quarter of birth instrument the estimates from the two stage least squares 2sls model using the unemployment rate and the model using the quarter of birth instruments are almost identical in addition these 2sls estimates are larger than the ordinary least squares ols estimates supporting this counterintuitive yet prevalent result in the literature

concepts for safety stock determination under stochastic demand and different types of random production yield abstract we consider a manufacturer s stochastic production inventory problem under periodic review and present methods for safety stock determination to cope with uncertainties that are caused by stochastic demand and different types of yield randomness following well proven inventory control concepts for this problem type we focus on a critical stock policy with a linear order release rule a central parameter of this type of policy is given by the safety stock value when non zero manufacturing lead times are taken into account in the random yield context it turns out that safety stocks have to be determined that vary from period to period we present a simple approach for calculating these dynamic safety stocks for different yield models additionally we suggest approaches for determining appropriate static safety stocks that are easier to apply in practice in a simulation study we investigate the performance of the proposed safety stock variants

safe port in a storm the impact of labor market conditions on community college enrollments the paper examines the impact of the business cycle on enrollments and finances at individual community colleges between the late 1960s and the mid 1980s we find that 1 percent increases in the unemployment rates of recent high school graduates and of all adults are associated with rises in full time attendance of about 0 5 percent and 4 percent respectively part time enrollment exhibits similar anticyclical patterns this link carries over in large part to degrees obtained in contrast state and local appropriations per student are procyclical we interpret this funding pattern as a failure to integrate education policy sufficiently closely with labor market policy

optimal policies for inventory systems with finite capacity and partially observed markov modulated demand and supply processes we analyze a single item periodic review inventory system with random yield and finite capacity operating in a random environment the primary objective is to extend the model of gallego and hu 2004 to the more general case when the environment is only partially observable although our analysis is specific to inventory systems it can also be applied to production systems by replacing the fixed capacity supplier with a fixed capacity producer using sufficient statistics we consider single period multiple period and infinite period problems to show that a state dependent modified inflated base stock policy is optimal moreover we show that the multiple period cost converges to the infinite period cost as the length of the planning horizon increases

integrating dynamic pricing and replenishment decisions under supply capacity uncertainty this paper examines an integrated decision making process regarding pricing for uncertain demand and sourcing from uncertain supply which are often studied separately in the literature our analysis of the integrated system suggests that the base stock list price policy fails to achieve optimality even under deterministic demand instead the optimal policy is characterized by two critical values a reorder point and a target safety stock under this policy a positive order is issued if and only if the inventory level is below the reorder point when this happens the optimal order and price are coordinated to achieve a constant target safety stock which aims at hedging the demand uncertainty we further investigate the profit improvement obtained from deploying dynamic pricing as opposed to static pricing our results indicate that either supply limit or supply uncertainty may induce a significant benefit from dynamic pricing and the compound effect of supply limit and uncertainty can be much more pronounced than the individual effects whether or not the supply capacity is limited has a major implication on the value of dynamic pricing under unlimited supply dynamic pricing is more valuable when procurement cost is high or when demand is more sensitive to price with limited supply however the capacity restriction tends to be relaxed reducing the value of dynamic pricing

analysis of a production inventory system subject to random disruptions in this paper we study an unreliable bottleneck production inventory system with a constant production and demand rate that is subject to random disruptions we assume that the restoration times are constant the time between breakdowns is exponential the production setup cost and or setup time is positive and excess demand is backordered we propose an s s production policy for such systems and develop expressions for the operating characteristics of the system the properties of the policy parameters that minimize the expected total cost rate are investigated and a procedure for finding their optimal values is developed in addition we devise and test a simple heuristic procedure for finding near optimal production policies the results of a numerical experiment suggest that i setup cost reduction is more effective in reducing total operating cost when the production system is more reliable ii setup cost reduction in unreliable production systems results in higher optimal safety stock level and iii proper determination of the safety stock levels is extremely important in just in time systems which are susceptible to disruptions

the effect of supply uncertainty in price setting newsvendor models abstract we consider a price setting newsvendor model in which a firm needs to make joint inventory and pricing decisions before the selling season the supply process is uncertain such that the received quantity is the product of the order quantity and a random yield rate two cost structures are investigated the in house production case in which the firm pays for the input quantity and the procurement case in which the firm pays for the quantity received only our objective is to investigate the effect of yield randomness on optimal decisions and expected profit by using the theory of stochastic comparisons we find that under both cost structures a less variable yield rate leads to a lower optimal price and a higher expected profit moreover we show that in the in house production case a stochastically larger yield rate also results in a lower optimal price and a higher profit but this is not true in the procurement case examples show that the effect of supply uncertainty on optimal order quantity is not universal

inventory inaccuracies in the wholesale supply chain abstract the inventory level shown in the information system contrary to popular belief and assumptions in most academic papers and in spite of the considerable amounts invested in information technology is often inaccurate the inventory inaccuracy occurs when the inventory shown in the information system is not in agreement with the actually available inventory in this paper we first describe the major factors generating inventory inaccuracy then we provide situations permitting to manage an inventory system subject to errors we provide a general framework permitting to model the inventory inaccuracy issue in particular we link the inaccuracy issue with the well known random yield problem the shown link permits us to derive the optimal ordering policy of an inventory framework where demand satisfaction is done based on the inventory records which are subject to inaccuracies we also propose an analysis permitting to show the added value of an advanced identification system such as the rfid technology

investment uncertainty and liquidity we analyze the dynamic investment decision of a firm subject to an endogenous financing constraint the threat of future funding shortfalls lowers the value of the firm s timing options and encourages acceleration of investment beyond the first best optimal level as well as highlighting another way by which capital market frictions can distort investment behavior this result implies that 1 the sensitivity of investment to cash flow can be greatest for high liquidity firms and 2 greater uncertainty has an ambiguous effect on investment

optimizing the level of renewable electric r d expenditures using real options analysis abstract one of the primary objectives of the united states federal non hydro renewable electric r d program is to promote the development of technologies that have the potential to provide consumers with stable and secure energy supplies in order to quantify the benefits provided by continued federal renewable electric r d this paper uses real option pricing techniques to estimate the value of renewable electric technologies in the face of uncertain fossil fuel prices within the real options analysis framework the current value of expected future supply from renewable electric technologies net of federal r d expenditures is estimated to be 30 6 billion of this value 86 can be attributed to past federal r d efforts and 14 can be attributed to future federal r d efforts assuming continued federal r d funding at 300 million yr in addition real options analysis shows that the value of renewable electric technologies increases as current and future r d funding levels increase this indicates that the current level of federal renewable electric r d funding is sub optimal

optimality of myopic ordering policies for inventory model with stochastic supply this paper addresses a discrete time inventory model where the maximum amount of supply from which instantaneous replenishment orders can be made is a random variable optimal ordering policies are characterized as critical number policies with monotone increasing optimal critical values a sufficient condition is then presented for myopic ordering policies to be optimal

stochastic comparisons in production yield management manufacturing firms routinely commit resources to increase yield rates through product and process improvement initiatives champions of such yield improvement projects may assume that stochastically larger yield rates are beneficial in this note we show that this need not hold even when the contingent production lot sizes are chosen optimally we employ stochastic comparison techniques to show that a yield rate that is smaller in the convex order ensures higher expected profit and we provide a distribution free bound on the size of increase in expected profit we also identify properties of yield rate distributions that do make stochastically larger yield rates beneficial

on the effect of demand randomness on inventories and costs we explore analytically cardinal effects of the extent of demand randomness on optimal inventory levels and the associated expected costs to model changes in demand randomness we make extensive use of a mean preserving transformation commonly used in probabilistic microeconomics as well as the notion of risk pooling aggregating independent demands for the single period news vendor model the order quantity and associated costs are shown to depend on the randomness parameter in a simple and highly interpretable manner we show by a simple example that in some situations risk pooling might neither reduce inventories nor move them closer to the mean or median demand the managerial relevance of such analyses is discussed

upper secondary school completion and the business cycle previous evidence suggests that enrollment in post compulsory education increases decreases in cyclical downturns upturns however little evidence exists on whether enrollment is successfully transformed into completed education this paper adds to the literature by analyzing the relationship between completion of upper secondary education and regional unemployment using norwegian regional panel data on students graduating from compulsory school between 1981 and 2004 we find robust evidence that completion rates are countercyclical our results suggest that poor labor market conditions when starting upper secondary education have a lasting effect and motivate students to stay in school and graduate

an approximate policy for a dual sourcing inventory model with positive lead times and binomial yield this paper studies the inventory system of a retailer who orders his products from two supply sources a local one that is responsive and reliable but expensive and a global one that is low cost but less reliable the deliveries from the global source only partially satisfy the quality requirements we model this situation with a dual sourcing inventory model with positive lead times and random yield we propose a dual index order up to policy dop based on approximating the inventory model with an unreliable supplier by a sequence of dual sourcing models with reliable suppliers and suitably modified demand distributions numerical results show that the performance of this heuristic is close to that of the optimal dop moreover we extend the heuristic to models with advance yield information and study its impact on the total inventory costs

existence of coordinating transshipment prices in a two location inventory model we consider a two location production inventory model where each location makes production decisions and is subject to uncertain capacity each location optimizes its own profits transshipment at a cost is allowed from one location to another we focus on the question of whether one can globally set a pair of coordinating transshipment prices i e payments that each party has to make to the other for the transshipped goods that induce the local decision makers to make inventory and transshipment decisions that are globally optimal a recent paper suggests for a special case of our model that there always exists a unique pair of coordinating transshipment prices we demonstrate through a counterexample that this statement is not correct and derive sufficient and necessary conditions under which it would hold we show that in some conditions coordinating prices may exist for only a narrow range of problem parameters and explore conditions when this can happen finally we study the effects of demand and capacity variability on the magnitude of coordinating transshipment prices

a non stationary periodic review production inventory model with uncertain production capacity and uncertain demand abstract in this paper we consider a nonstationary periodic review dynamic production inventory model with uncertain production capacity and uncertain demand the maximum production capacity varies stochastically it is known that order up to or base stock critical number policies are optimal for both finite horizon problems and infinite horizon problems we obtain upper and lower bounds of the optimal order up to levels and show that for an infinite horizon problem the upper and the lower bounds of the optimal order up to levels for the finite horizon counterparts converge as the planning horizons considered get longer furthermore under mild conditions the differences between the upper and the lower bounds converge exponentially to zero

a real option analysis of investments in hydropower the case of norway abstract this paper presents a valuation study of hydropower investment opportunities in the norwegian context according to nve norwegian water resources and energy directorate the regulator there is a potential of 39 twh not yet developed generation in a normal year is approximately 120 twh by using the conceptual real option framework of dixit and pindyck 1994 investment under uncertainty princeton university press princeton nj one can estimate the value of investment opportunities to nok 11 million gwh eur 1 4 million gwh furthermore the optimal trigger price for initiating an investment based on electricity forward prices is calculated to nok 0 32 kwh eur 0 04 kwh the analysis shows consistency between real option theory and aggregate investment behaviour in norwegian hydropower

optimal joint inventory and transshipment control under uncertain capacity in this paper we address the optimal joint control of inventory and transshipment for a firm that produces in two locations and faces capacity uncertainty capacity uncertainty e g due to downtime quality problems yield etc is a common feature of many production systems but its effects have not been explored in the context of a firm that has multiple production facilities we first characterize the optimal production and transshipment policies and show that uncertain capacity leads the firm to ration the inventory that is available for transshipment to the other location and characterize the structure of this rationing policy then we characterize the optimal production policies at both locations which are defined by state dependent produce up to thresholds we also describe sensitivity of the optimal production and transshipment policies to problem parameters and in particular explain how uncertain capacity can lead to counterintuitive behavior such as produce up to limits decreasing for locations that face stochastically higher demand we finally explore through a numerical study when the optimal policy is most likely to yield significant benefits compared to simple policies

infinite horizon strategies for replenishment systems with a general pool of suppliers we consider a general infinite horizon inventory control model that combines demand and supply risks and the firm s ability to mitigate the supply risks by diversifying its procurement orders among a set of n potential suppliers supply risks arise because only a random percentage of any given replenishment order is delivered as useable units the suppliers are characterized by the price they charge and the distribution of their yield factor assuming unsatisfied demand is backlogged the firm incurs as in standard inventory models three types of costs i procurement costs ii inventory carrying costs for units carried over from one period to the next and iii backlogging costs we establish the existence of an optimal stationary policy under both the long run discounted and average cost criteria and characterize its structure assuming each period s inventory level distribution can be approximated as a normal we develop an efficient solution method identifying additional structural properties finally we identify a simple class of heuristic policies that come close to being optimal

does compulsory school attendance affect schooling and earnings we establish that season of birth is related to educational attainment because of school start age policy and compulsory school attendance laws individuals born in the beginning of the year start school at an older age and can therefore drop out after completing less schooling than individuals born near the end of the year roughly 25 percent of potential dropouts remain in school because of compulsory schooling laws we estimate the impact of compulsory schooling on earnings by using quarter of birth as an instrument for education the instrumental variables estimate of the return to education is close to the ordinary least squares estimate suggesting that there is little bias in conventional estimates every developed country in the world has a compulsory schooling requirement yet little is known about the effect these laws have on educational attainment and earnings 1 this paper exploits an unusual natural experiment to estimate the impact of compulsory schooling laws in the united states the experiment stems from the fact that children born in different months of the year start school at different ages while compulsory schooling laws generally require students to remain in school until their sixteenth or seventeenth birthday in effect the interaction of school entry requirements and compulsory schooling laws compel students born

income schooling and ability evidence from a new sample of identical twins we develop a model of optimal schooling investments and estimate it using new data on approximately 700 identical twins we estimate an average return to schooling of 9 percent for identical twins but estimated returns appear to be slightly higher for less able individuals simple cross section estimates are marginally upward biased these empirical results imply that abler individuals attain more schooling because they face lower marginal costs of schooling not because of higher marginal benefits

supply and demand functions in inventory models the central issue in supply chain management is to match supply with demand and the heart of a planning model is the modeling of supply and demand functions to allow for analytical tractability the existing literature often assumes almost surely linear supply and demand functions which greatly limits the applicability of the models the goal of this paper is to provide a unified approach to analyze general random supply and demand functions by transforming the problem into one defined on a higher dimension we show that many of the seemingly highly nonlinear supply and demand functions in the almost sure sense are linear in the stochastic sense with this new notion of linearity called stochastic linearity in midpoint our ability to analyze supply chain problems is much enhanced we are able to prove the concavity of the profit function in the transformed supply and demand decisions for a general class of supply and demand functions that include but are not restricted to the ones studied in the

explicit investment rules with time to build and uncertainty abstract we establish explicit socially optimal rules for an irreversible investment decision with time to build and uncertainty assuming a price sensitive demand function with a random intercept we provide comparative statics and economic interpretations for three models of demand arithmetic brownian geometric brownian and the cox ingersoll ross committed capacity that is the installed capacity plus the investment in the pipeline must never drop below the best predictor of future demand minus two biases the discounting bias takes into account the fact that investment is paid upfront for future use the precautionary bias multiplies a type of risk aversion index by the local volatility relying on the analytical forms we discuss in detail the economic effects for example the impact of volatility on the optimal investment is negligible in some cases it vanishes in the cir model for long delays and in the gbm model for high discount rates

tactical inventory and backorder decisions for systems with predictable production yield abstract we consider a manufacturing system with stochastic demand and predictable production yield the manufacturer has predetermined prices and limited production capacity in each period the manufacturer also has the option to save some inventory for future periods even if there is demand in the current period the demand that is not met is lost or may be backordered for only one period our objective is to maximize the expected profit by choosing optimal produce up to level y t save s t and backorder quantities b t in each period t we formulate this problem as a markov decision process where the state of the system is represented by the net inventory and the yield rate we show that a modified order up to policy y t s t b t is optimal in each period t we also perform computational analysis to examine the effects of production yield on the optimal decisions

multiple lot sizing and value of probabilistic information in production to order of an uncertain size abstract a make to order batch manufacturer knows that an order for items with some particular specifications is forthcoming but does not yet know the exact size of the order due to production lead times work needs to start immediately but some units produced may fail to meet required tolerances that is the yield of each batch is random each new run involves a costly setup and each unit attempted involves material and other variable costs if a subjective probability distribution for the order size can be formed the following related questions arise 1 a what is the optimal run size 2 b when should production stop to answer these questions this paper formulates a new model whose main new feature is the ability to handle uncertain demand within a multiple lot sizing setting it is proved that the optimal policy for this scenario is one of control limit stop if and only if the stock of good units is larger than some critical value a computer program was developed for solving the problem for binomial yields and optimal policies for several examples are reported due to the effort required to form and communicate probabilistic forecasts a common practice is to express a forecast as a single number corresponding perhaps to the mean or median of the unarticulated distribution we compute the extra profit expected from forming and using probabilistic forecasts we also show how to compute the profit s variance for any policy

technical note linear inflation rules for the random yield problem analysis and computations in this paper we propose a simple heuristic approach for the inventory control problem with stochastic demand and multiplicative random yield our heuristic tries to find the best candidate within a class of policies that are referred to in the literature as the linear inflation rule lir policies our approach is computationally fast easy to implement and intuitive to understand moreover we find that in a significant number of instances our heuristic performs better than several other well known heuristics that are available in the literature

production control in the face of storable raw material random supply and an outside market we study a periodic review production inventory control problem where both the supply of raw material and demand for the finished product are exogenous and random the raw material can be stored for future use can be purchased from or sold to an outside market we study both the lost sales and backlogging cases under both strict convex and linear raw material purchasing selling costs convexity of the purchasing selling cost implies that the more the firm purchases from or sells to the outside market the more expensive or less valuable the raw material becomes for all cases we find the partial characterizations for the optimal policies which all turn out to be very intuitively appealing in particular for each of the linear cost cases the optimal policy degenerates into a combination of two base stock policies one for the raw material inventory and the other for the finished product inventory

time to build option value and investment decisions abstract investment decisions and outlays are often made sequentially for example the rate at which construction proceeds is usually flexible and can be adjusted with the arrival of new information traditional discounted cash flow methods which treat the pattern of investment as fixed ignore this flexibility and understate the value of the project this paper uses contingent claims analysis to derive optimal decision rules and to value such investments we determine the effects of time to build opportunity cost and uncertainty on the investment decision for reasonable parameter values we show how a simple npv rule can lead to gross errors

periodic review inventory systems with fixed order cost and uniform random yield abstract we consider periodic review stochastic inventory control problems with both fixed order cost and uniform random yield our objective is to characterize some structural properties of the optimal policies so that efficient approaches can then be established towards finding an optimal policy in particular we provide a lower and upper bound structure for the optimal policy at the beginning of any period such that it is optimal not to order anything if the initial stock level is above the upper bound and it is optimal to order a positive quantity if the initial stock level is below the lower bound where the optimal quantity can be found efficiently for any initial stock level in between the upper and lower bounds in each period a partial characterization of the optimal policy is provided furthermore we show that the optimal order quantity is monotonically decreasing in the initial stock level in any period and this will help the search for the optimal order quantity at any initial stock level in addition we illustrate a few interesting phenomena about the behavior of an optimal policy these phenomena show that the structure of the optimal policy for this problem is in general significantly different from that for the traditional periodic review inventory control problems with certain yield and fixed order cost

analytical solution for an investment problem under uncertainties with shocks abstract we derive the optimal investment decision in a project where both demand and investment cost are stochastic processes eventually subject to shocks we extend the approach used in dixit and pindyck 1994 to deal with two sources of uncertainty and we assume that the underlying processes are jump diffusion processes assuming certain conditions on the parameters we are able to derive a closed expression for the value of the firm finally we present comparative statics for the investment threshold with respect to the relevant parameters

base stock policies for production inventory problems with uncertain capacity levels abstract in this paper we consider a single item stochastic demand production inventory problem where the maximum amount that can be produced or ordered in any given period is assumed to be uncertain inventory levels are reviewed periodically the system operates under a stationary modified base stock policy the intent of our paper is to present a procedure for computing the optimal base stocl level of this policy under expected average cost per period criterion this procedure would provide guidance as to the appropriate amount of capacity to store in the form of inventory in the face of stochastic demand and uncertain capacity in achieving this goal our main contribution is to establish the analogy between the class of base stock production inventory policies that operate under demand capacity uncertainty and the g g 1 queues and their associated random walks we also present example derivations for some important capacity distributions

a periodic review production planning model with uncertain capacity and uncertain demand optimality of extended myopic policies increasing product complexity manufacturing environment complexity and an increased emphasis on product quality are all factors leading to uncertainties in production processes these uncertainties are in the form of unplanned machine maintenance varying production yields and rework among others in planning for production an adequate model must incorporate these uncertainties into the representation of the production process this paper treats the aggregate planning problem for a single product with random demand and random capacity in the single period problem random capacity does not affect the optimal policy but results in a unimodal nonconvex cost function in the multiple period and infinite horizon settings order up to policies that are dependent on the distribution of capacity are shown to be optimal in spite of a nonconvex cost in the infinite horizon setting an intuitive description of the situation leads to the notion of a class of extended myopic policies requiring the consideration o

product portfolio management with production flexibility in agribusiness the acquisition of production flexibility is a well documented strategy pursued by many firms to counteract certain operational constraints however these flexibilities can increase the complexity of a production system and the difficulties in managing increased complexity may hinder exploiting the full benefit of flexibility in this paper we consider one such flexibility paradox at an agribusiness firm for an annual 800 million production decision the firm produces a number of products hybrid seeds using limited inventories of several raw materials parent seeds and a production process that is subject to random variations to handle the raw material availability constraint and to partially mitigate the supply risk the firm invests in a costly second production in south america that can be used if the yield in the first production in north america is low we solve this joint problem of raw material allocation and sequential production by reformulating it as a tractable simultaneous optimization

joint inventory replenishment and pricing control for systems with uncertain yield and demand we study the joint inventory replenishment and pricing problem for production systems with random demand and yield more specifically we analyze the following single item periodic review model demands in consecutive periods are independent random variables and their distributions are price sensitive the production yield is uncertain so that the quantity received from a replenishment is a random variable whose distribution depends on the production quantity stockouts are fully backlogged our problem is to characterize the optimal dynamic policy that simultaneously determines the production quantity and the price for each period to maximize the total discounted profit we show that the optimal replenishment policy is of a threshold type i e it is optimal to produce if and only if the starting inventory in a period is below a threshold value and that both the optimal production quantity and the optimal price in each period are decreasing in the starting inventory we further study the operational e

evaluation of cycle count policies for supply chains with inventory inaccuracy and implications on rfid investments inventory record inaccuracy leads to ineffective replenishment decisions and deteriorates supply chain performance conducting cycle counts i e periodic inventory auditing is a common approach to correcting inventory records it is not clear however how inaccuracy at different locations affects supply chain performance and how an effective cycle count program for a multi stage supply chain should be designed this paper aims to answer these questions by considering a serial supply chain that has inventory record inaccuracy and operates under local base stock policies a random error representing a stock loss such as shrinkage or spoilage reduces the physical inventory at each location in each period the errors are cumulative and are not observed until a location performs a cycle count we provide a simple recursion to evaluate the system cost and propose a heuristic to obtain effective base stock levels for a two stage system with identical error distributions and counting costs we prove that it is more effective to conduct more frequent cycle counts at the downstream stage in a numerical study for more general systems we find that location proximity to the customer error rates and counting costs are primary factors that determine which stages should get a higher priority when allocating cycle counts however it is in general not effective to allocate all cycle counts to the priority stages only one should balance cycle counts between priority stages and non priority stages by considering secondary factors such as lead times holding costs and the supply chain length in particular more cycle counts should be allocated to a stage when the ratio of its lead time to the total system lead time is small and the ratio of its holding cost to the total system holding cost is large in addition more cycle counts should be allocated to downstream stages when the number of stages in the supply chain is large the analysis and insights generated from our study can be used to design guidelines or scorecard systems that help managers design better cycle count policies finally we discuss implications of our study on rfid investments in a supply chain

a unified theory of the dynamics of closed loop supply chains we investigate the dynamics of a closed loop supply chain with first order auto regressive ar 1 demand and return processes we assume these two processes are cross correlated the remanufacturing process is subject to a random triage yield remanufactured products are considered as goodas new and used to partially satisfy market demand newly manufactured products make up the remainder we derive the optimal linear policy in our closed loop supply chain setting to minimise the manufacturer s inventory costs we show that the lead time paradox can emerge in many cases in particular the auto and cross correlation parameters and variances of the error terms in the demand and the returns as well as the remanufacturing lead time all influence the existence of the lead time paradox finally we propose managerial recommendations for manufacturers

the impact of wages and unemployment on youth enrollment and labor supply various aspects of the school enrollment labor supply decision have been examined in earlier studies including those of bowen and finegan 1969 cohen rea and lerman 1970 duncan 1965 katz 1973 korbel 1966 and lerman 1972 although these authors were aware of the joint nature of the school work decision typically they used an ordinary least squares ols framework with dichotomous dependent variables to analyze enrollment conditional on the labor supply decision or labor supply conditional on enrollment several more recent studies such as antos and mellow 1978 stephenson 1978 and ehrenberg and marcus forthcoming use multinomial logit to analyze the joint determination of labor supply and enrollment mallar 1976 uses a bivariate probit model to deal with the simultaneous relation between school and work but none of these studies considers both the wage offers and job availability there is still available no single set of estimates which considers the roles of both job availability and the wage on the joint enrollment labor supply decision consider what the interrelation between wages and job availability implies for estimation of the labor force participation school enrollment decision to the extent that downwardly rigid wages prevent the youth labor market from clearing one would expect both wages and job availability to influence labor supply if one of these measures is not included in explaining labor supply the coefficient estimated for the other measure may for reasons well known be biased for example if the wage is omitted but the unemployment variable included and if wages and unemployment are negatively correlated then the magnitude of the coefficient of the unemployment variable will be increased it is also unclear on a priori grounds how job availability affects school enrollment on the one hand if jobs are readily available to young people youth from poor families may be able to enroll in school supporting themselves through part time work with no jobs available some may be unable to afford school expenditures and may drop out bowen and finegan 1969 p 404 on the other hand readily available employment opportunities for youth may simply raise the probability of dropping out of school and working full time our findings will help to determine how young people react to this influence and to other influences of the market from the viewpoint of public policy there are a number of reasons why it is important that the enrollment labor supply decisions be understood and that the separate responses to wage and job availability be isolated for one thing there is a tendency to discount the welfare importance of unemployment or poor labor participation for young people enrolled in school feldstein and ellwood 1979 however the joint nature of the enrollment labor supply decisions suggests that a high enrollment rate may not be simply a cause of low participation rates but may be an additional symptom of adverse labor market conditions indeed our results for nonwhite males support an interpretation of this kind second the joint nature of the decisions also implies that public programs such as the received for publication january 31 1980 revision accepted for publication december 11 1980 both authors are with dartmouth college and the national bureau of economic research the research reported here was supported by the division of technical systems in the office of technical and analytic systems office of the assistant secretary for planning and budget u s department of education and is part of the nber s research program in labor studies any opinions expressed are those of the authors and not those of the national bureau of economic research or of the department of education we would like to thank gary fields meir kohn robert plotnick martin segal and participants in the labor seminars at harvard and at cornell for their helpful comments

the effect of firm level contracts on the structure of wages evidence from matched employer employee data in spain as in several other european countries sectoral bargaining agreements are automatically extended to cover all firms in an industry employers and employees can also negotiate firm specific contracts we use a large matched employer employee data set to study the effects of firm level contracting on the structure of wages employees covered by firm specific contracts earn about 10 percent more than those covered by sectoral contracts the estimated premium is about the same for men in different skill groups but higher for more highly skilled women suggesting that firm level contracts raise wage inequality for women at the establishment level we compare average wages under firm level and sectoral bargaining controlling for the propensity to negotiate a firm specific contract consistent with the worker level models we find that firm specific contracting raises average wages with a pattern of effects that tends to increase inequality relative to sectoral bargaining for women although we cannot decisively test between alternative explanations for the firm level contracting premium workers with firm specific contracts have significantly longer job tenure suggesting that the premium is at least partially a non competitive phenomenon

a note on coordination in decentralized assembly systems with uncertain component yields abstract gurnani and gerchak h gurnani y gerchak coordination in decentralized assembly systems with uncertain component yields european journal of operational research 176 2007 1559 1576 study coordination of a decentralized assembly system in which the demand of the assembler is deterministic and the component yields are random they present incentive alignment control mechanisms under which system coordination is achieved in this note we extend gurnani and gerchak s model to the case of positive salvage value and n asymmetric suppliers and show that the shortage penalty contract which can coordinate gurnani and gerchak s model no longer coordinates the extended model furthermore we present a new kind of contract surplus subsidy contract to coordinate the extended model and prove that the profit of the supply chain under coordination can be arbitrarily divided between the component suppliers and the assembler

the hidden costs of renewables promotion the case of crop based biogas abstract this paper finds that specific characteristics of bioenergy investments namely long times to build relatively high variable costs and the managerial ability of corrective actions during the project help bioenergy producers to respond to market uncertainties this effect is shown to compromise the economic effectiveness of bioenergy policies the paper compares the effectiveness of financial and regulatory incentive programs aiming to stimulate production of crop based biogas a stylized stochastic real options based investment model is developed to analyze the investment behavior of biogas producers under alternative policies the model is solved by a numerical approach combining stochastic simulations and the genetic algorithm technique unless bioenergy schemes account for structural characteristics of the bioenergy industry they will tend to encourage investments in unused production capacities this would cause additional welfare costs and benefit mainly crop producers

analysis of an inventory system under supply uncertainty in this paper we analyze a periodic review single item inventory model under supply uncertainty the objective is to minimize expected holding and backorder costs over a finite planning horizon under the supply constraints the uncertainty in supply is modeled using a three point probability mass function the supply is either completely available partially available or the supply is unavailable machine breakdowns shortages in the capacity of the supplier strikes etc are possible causes of uncertainty in supply we demonstrate various properties of the expected cost function and show the optimality of order up to type policies using a stochastic dynamic programming formulation under the assumption of a bernoulli type supply process in which the supply is either completely available or unavailable and when the demand is deterministic and dynamic we provide a newsboy like formula which explicitly characterizes the optimal order up to levels an algorithm is given that computes the optimal inventory levels over the planning horizon extensions and computational analysis are presented for the case where the partial supply availability has positive probability of occurrence

hedging the value of waiting abstract we analyze the optimal hedging policy of a firm that has flexibility in the timing of investment conventional wisdom suggests that hedging adds value by alleviating the under investment problem associated with capital market frictions however our model shows that hedging also adds value by allowing investment to be delayed in circumstances where the same frictions would cause it to commence prematurely thus hedging can have the paradoxical effect of reducing investment we also show that greater timing flexibility increases the optimal quantity of hedging but has a non monotonic effect on the additional value created by hedging these results may help explain the empirical findings that investment rates do not differ between hedgers and non hedgers and that hedging propensities do not depend on standard measures of growth opportunities

the capacitated newsboy problem with random yield the gardener problem abstract this publication is the third in a series of articles by abdel malek et al 2004 exact approximate and generic iterative models for the multi product newsboy with budget constraint international journal of production economics 91 189 198 and abdel malek and montanari 2005 an analysis of multi product newsboy problem with a budget constraint international journal of production economics 97 296 307 which appeared in ijpe addressing issues regarding the newsboy models in this paper models are developed to extend the existing ones to cover random yield scenarios we designate this type of models as the gardener problem the models are based on the application of lagrange multipliers leibniz s rule and newton s method to obtain the optimum solution for the considered random yield and probabilistic demand situations the developed methodologies are applicable to general probability distribution functions examples are given to show the applicability of the developed approach to different probability distributions for both the supply and the demand as well as for interdependent yield structures

sub supply chain coordination in a three layer chain under demand uncertainty and random yield in production abstract one of the major objectives of modern supply chain management is to deal with the growing decentralization among the involved entities and hence minimizing the double marginalization effect inside the chain especially when the end customers demand is not deterministic to address this a three layer supply chain with one raw material supplier one manufacturer and one retailer is developed and studied in this paper effects of both supply chain coordination as well as sub supply chain coordination are examined optimal order and production quantities are obtained for the centralized system which depicts the scenario under coordination optimal order quantities and expected profits of the individual channel actors are maximized locally in the decentralized system under commonly used price only contract semi integrated models are also studied under price only contract the optimal strategies under different power structures are compared and the effects of the channel parameters on the optimal strategies are also examined numerical example is given to illustrate the developed model

selecting a portfolio of suppliers under demand and supply risks we analyze a planning model for a firm or public organization that needs to cover uncertain demand for a given item by procuring supplies from multiple sources each source faces a random yield factor with a general probability distribution the model considers a single demand season all supplies need to be ordered before the start of the season the planning problem amounts to selecting which of the given set of suppliers to retain and how much to order from each so as to minimize total procurement costs while ensuring that the uncertain demand is met with a given probability the total procurement costs consist of variable costs that are proportional to the total quantity delivered by the suppliers and a fixed cost for each participating supplier incurred irrespective of his supply level each potential supplier is characterized by a given fixed cost and a given distribution of his random yield factor the yield factors at different suppliers are assumed to be independent of the season s demand which is described by a general probability distribution determining the optimal set of suppliers the aggregate order and its allocation among the suppliers on the basis of the exact shortfall probability is prohibitively difficult we have therefore developed two approximations for the shortfall probability although both approximations are shown to be highly accurate the first based on a large deviations technique ldt has the advantage of resulting in a rigorous upper bound for the required total order and associated costs the second approximation is based on a central limit theorem clt and is shown to be asymptotically accurate whereas the order quantities determined by this method are asymptotically optimal as the number of suppliers grows most importantly this clt based approximation permits many important qualitative insights

analysis of an inventory system under backorder correlated deterministic demand and geometric supply process abstract in this article we propose a single item periodic review model that investigates the effects of changes in the demand process that occur after stockout realizations we investigate a system where the demands in successive periods are deterministic but affected by the backorder realizations in order to capture the effects of changes in the demand process we use a geometric type supply availability we analytically derive the necessary components for obtaining profit related performance measures and provide computational analysis

analysis of a q r t inventory policy with deterministic and random yields when future supply is uncertain abstract this paper considers a stochastic inventory model where the quantity ordered sometimes may not be available due to strikes etc we represent the supplier s availability process as a two state continuous time markov chain where one state corresponds to availability and the other state corresponds to unavailability of the supplier the problem is to determine the reorder point the order quantity when the system is found in on state and how long to wait before the next order if system is in off state it is assumed that the state of the system is identified at a cost using the renewal reward theorem we construct the objective function as the long run average cost numerical sensitivity analysis results are provided we also analyze the problem when the yield i e amount received if available is random and discuss an example with beta distributed yield

periodic review production models with variable capacity random yield and uncertain demand we investigate a production planning problem in a periodic review environment with variable production capacity random yields and uncertain demand the implications of random yields and variable capacity for lot sizing previously have been explored separately but not jointly many production environments are likely to be subject to both types of uncertainties to minimize the total discounted expected costs production holding and shortage costs we formulate the problem as a stochastic dynamic program for the finite horizon problem we prove that the objective function is quasi convex and that the structure of the optimal policy is characterized by a single critical point for the initial stock level at each period that is if the initial stock is greater than this critical point the optimal planned production is zero otherwise it is greater than zero expressions for solving the critical point and the optimal planned production are obtained we further show that the solution for the finite horizon problem converges to that of the infinite horizon problem

exact and heuristic linear inflation policies for an inventory model with random yield and arbitrary lead times abstract we investigate a periodic review inventory system for a single item with stochastic demand and random yield since the optimal policy for such a system is complicated we study the class of stationary linear inflation policies where orders are only placed if the inventory position is below a critical stock level and where the order quantity is controlled by a yield inflation factor we consider two different models for the uncertain supply namely binomial and stochastically proportional yield and we allow positive and constant lead times as well as asymmetric demand and yield distributions in this paper we propose two novel approaches to derive optimal and near optimal numerical values for the critical stock level minimizing the average holding and backorder cost for a given inflation factor first we present a markov chain approach which is exact in case of negligible lead time second we provide a steady state analysis to derive approximate closed form expressions for the optimal critical stock level we conduct an extensive numerical study to test the performance of our approaches the numerical experiments reveal an excellent performance of both approaches since our derived formulas are easily implementable and highly accurate they are very valuable for practical application

leadtime management in a periodic review inventory system a state dependent base stock policy we examine a single item periodic review inventory system with stochastic leadtimes in which a replenishment order is delivered immediately or one period later depending probabilistically on costly effort the objective is to determine a joint inventory policy and effort choice strategy that minimizes the expected total costs our analytical and computational analysis suggests that i a state dependent base stock policy is optimal ii the optimal effort strategy is such that the marginal cost of effort is equal to the value of immediate delivery and iii the cost impact of leadtime reduction can be very large we also provide some counter intuitive results compared with the traditional multi period newsvendor model

interpreting instrumental variables estimates of the returns to schooling this article synthesizes economic insights from theoretical models of schooling choice based on individual benefits and econometric work interpreting instrumental variables estimates as weighted averages of individual specific causal effects linkages are illustrated using college proximity to instrument for schooling after characterizing groups differentially affected by the instrument according to family background i directly compute weights underlying estimation of the overall return in analyzing the level of schooling at which individuals change their behavior in response to the instrument i demonstrate that this instrument has its greatest impact on the transition from high school to college specification robustness is also examined

random yield supply chain with a yield dependent secondary market abstract this paper considers a simple supply chain with one supplier and one retailer where the supplier s production is subject to random yield and the retailer faces uncertain demand there exists a secondary market for acquiring or disposing products by the supplier we study both the centralized and decentralized systems in the decentralized system a no risk sharing contract and a risk sharing minimum commitment contract are analyzed the supply chain with the risk sharing contract is further analyzed with a constant secondary market price and a yield dependent secondary market price we present both the supplier s and the retailer s optimal strategies and provide insights for managers when making decisions under random yield risk and demand uncertainty we find that the secondary market generally has a positive impact on supply chain performance and the actual effect of random yield risk on the supply chain performance depends on cost parameters and supply chain contract settings under certain conditions reducing yield randomness may weaken the double marginalization effect and improve the chain performance from the numerical study we also show that there exists an optimal commitment level for the supply chain

multiperiod sales production decisions under uncertainty abstract this paper studies the behavior of an expected profit maximizing multiperiod firm under uncertainty in output and demand the firm s problem is to determine in each period the level of sales and inventory and the expected output for the coming period the optimal policy is derived in an explicit closed form and is proved to be myopic it is shown that the observed phenomenon of downward price inflexibility can be explained and further specified by using the results of our model

the structure of periodic review policies in the presence of random yield this work provides a comprehensive analysis of a general periodic review production inventory model with random variable yield existence of an order point whose value does not depend on yield being random is proved in the single period case without specifying the yield model and using a very general cost structure when yield is a random multiple of lot size the nonorder up to optimal policy is characterized for a finite horizon model the finite horizon value functions are shown to converge to the solution of an infinite horizon functional equation and the infinite horizon order point is shown to be no smaller than when yield is certain

a single period stocking and pricing problem involving stochastic emergency supply abstract we develop a model for determining jointly optimal price and production policy of products with short life cycles subject to partially uncertain supply capacity within the price setting newsvendor framework we consider that demand in excess of the stocking quantity can be met by an emergency order due to the randomness of emergency supply capacity it may not always be possible to satisfy the demand completely assuming that all customers are charged the same price we explore the optimal sourcing and pricing decisions in the commonly used additive and multiplicative demand scenarios the first order conditions imply that the optimal price in the problem with random emergency supply is less than the optimal price associated with the deterministic demand problem when demand uncertainty is additive and it is greater than the optimal price associated with the deterministic demand problem when demand uncertainty is multiplicative we also find that the same type of comparative static applies in the multiplicative uncertainty case when there exists a dual source emergency system consisting of a perfectly reliable supplier and a second supplier with a stochastic capacity

inventory control with markovian capacity and the option of order rejection we consider a stochastic inventory control problem with markovian capacity and the option of order rejection we show its optimal policy to be the combination of a capacity dependent modified base stock production policy and a capacity dependent critical point order acceptance policy when capacity is stochastically monotone we show that the policy parameters change over the current capacity level in an intuitive way all these results can be extended to the infinite horizon case with or without cost discounting our computational study substantiates the benefits from both exploiting the markovian behavior of the capacity and rejecting orders when necessary

the impact of youth unemployment on adult unemployment in the ncds using the national child development survey this paper looks at cumulated experience of unemployment highlighting how unemployment experience is concentrated on a minority of the workforce over extended periods low educational attainment ability not captured by education financial deprivation and behavioural problems in childhood raise a person s susceptibility to unemployment there is strong evidence of structural dependence induced by early unemployment experience for men but only minor persistence for women attacking low educational achievement and preventing the build up of substantial periods in unemployment as youths may reduce the extent to which a minority of men spend a large part of their working lives unemployed

optimality of myopic policies for dynamic lot sizing problems in serial production lines with random yields and autoregressive demand we study lot size policies in a serial multistage manufacturing inventory system with two key generalizations namely 1 random yields at each production stage and 2 an autoregressive demand process previous research shows that the optimal policies in models with random yields even in models with a single installation lack the familiar order up to structure and are not myopic thus dynamic programming algorithms are needed to compute optimal policies and one encounters the curse of dimensionality this is exacerbated here by the need to expand the size and dimension of the state space to accommodate the autoregressive demand feature nevertheless although our model is more complex we prove that there is an optimal policy with the order up to feature and more importantly that the optimal policy is myopic this avoids the computational burden of dynamic programming our results depend on two assumptions concerning the stochastic yield namely that the expected yield at a work station is proportional to the lot size and the distribution of the deviation of the yield from its mean does not depend on the lot size we introduce the concept of echelon like variables a generalization of clark and scarf s classical concept of echelon variables to derive the structure of optimal policies furthermore we show that the same kind of policy is optimal for several criteria infinite horizon discounted cost infinite horizon long run average cost and finite horizon discounted cost with the appropriate choice of the salvage value function

myopic heuristics for the random yield problem we consider a single item periodic review inventory problem with random yield and stochastic demand the yield is proportional to the quantity ordered with the multiplicative factor being a random variable the demands are stochastic and are independent across the periods but they need not be stationary the holding penalty and ordering costs are linear any unsatisfied demands are backlogged two cases for the ordering cost are considered the ordering cost can be proportional to either the quantity ordered e g in house production or the quantity received e g delivery by an external supplier random yield problems have been addressed previously in the literature but no constructive solutions or algorithms are presented except for simple heuristics that are far from optimal in this paper we present a novel analysis of the problem in terms of the inventory position at the end of a period this analysis provides interesting insights into the problem and leads to easily implementable and highly accurate myopic heuristics a detailed computational study is done to evaluate the heuristics the study is done for the infinite horizon case with stationary yields and demands and for the finite horizon case with a 26 period seasonal demand pattern the best of our heuristics has worst case errors of 3 0 and 5 0 and average errors of 0 6 and 1 2 for the infinite and finite horizon cases respectively

environmental safety stock the impacts of regulatory and voluntary control policies on production planning inventory control and environmental performance abstract this paper analyzes the impacts of different pollution control policies on a firm s decisions of production planning and inventory control based on a stochastic model with both demand and environmental uncertainties we derive the optimal policies of production planning and inventory control under both regulatory and voluntary pollution control approaches and investigate their operational and environmental effects we establish that the conventional wisdom which suggests that reduction of environmental waste at the end of a production process also decreases the stock and throughput levels of a production system is not necessarily true rather a regulatory environmental standard that limits the total amount of waste may induce the firm to raise its planned stock level which would lead to a higher expected amount of environmental wastes before the standard is enforced as well as environmental risks at other stages of the production process the additional planned stock level which is termed environmental safety stock can be reversed by using the voluntary control approach that provides the firm with the flexibility to occasionally exceed the environmental standard we also conduct numerical experiments to analyze the effects of different values of model parameters under different control approaches the analytical results provide new insights to the impacts of a firm s production and inventory decisions on the natural environment as well as to the choices of pollution control approaches by decision makers in both the private and public sectors

inventory models with random yield in a random environment abstract we consider a single item inventory model which is observed periodically in a randomly changing environment all model parameters are specified by the state of the environment which is assumed to be a time homogeneous markov chain yield is random due to the random capacity of the vendor i e a given order is fully received if the order quantity is less than this capacity otherwise the quantity received is equal to the available capacity the problem is analyzed in single multiple and infinite periods and it is shown that in all cases the optimal policy is the well known base stock policy where the optimal order up to level depends on the state of the environment the results are compared with the solutions of the certain yield model when there is infinite capacity we show that the order up to levels are equal in the single period case however in multiple and infinite periods we order the same or more if the yield is random

what do twins studies reveal about the economic returns to education a comparison of australian and us findings conventional estimates of rates of return to education are constrained because they are unable to isolate the returns to schooling from the contribution of individual ability and the influence of family background potentially studies using a sample of twins may overcome these problems monozygotic twins are genetically identical and if they have been reared together share the same family background differences in income between identical twins can therefore be associated with differences in the amount of education they have undertaken in order to estimate the independent influence of education comparisons of such estimates with estimates made from a sample of dizygotic twins who are genetically similar but not identical and who are also reared together may permit evaluation of the biases associated with ability and family background in conventional estimates of the return to education in an important paper jere behrman et al 1977 analyzed data from a sample of male world war ii veteran twins and found that of the overall rate of return to schooling of 8 percent 2 7 percentage points could be attributed to schooling per se 3 2 percentage points could be attributed to genetic factors and 2 1 percentage points could be attributed to shared family environment in a recent paper however orley ashenfelter and alan krueger 1994 have estimated the economic returns to education using data on a new sample of twins which permitted them to adjust their estimate for omitted ability variables and measurement error on this basis they find that family and genetic effects make virtually no contribution to the returns to schooling measurement error however biases the estimated return to schooling downward the findings of ashenfelter and krueger 1994 are very different from previous work in this field and there is now some confusion as to what analysis of twins data really reveals about the economic returns to schooling ashenfelter and krueger s results are based on analysis of data for a sample of 298 individuals 149 pairs of identical twins who attended a twins festival in 1991 they remark at the end of their paper only additional data collection is likely to lead to better estimates of the returns to schooling p 1171

endowments and the allocation of schooling in the family and in the marriage market the twins experiment 03 we show how comparisons between the within twin correlations of human capital outcomes across identical and nonidentical twins can be used to identify the variability in the individual specific component of endowments and the responsiveness of schooling to individual specific endowments in the family and in the marriage market even when schooling is measured with error estimates from two twins samples indicate that 27 42 percent of the variance in log earnings obesity is due to variability in individual specific endowments allocations of schooling reinforce specific endowments and individual specific earnings endowments of men and their wives schooling are negatively associated

estimates of the returns to schooling from sibling data fathers sons and brothers data on brothers and on fathers and sons from the national longitudinal survey are used to consider the impact of omitted variables and measurement errors on the economic returns to schooling the analysis suggests that the upward bias in estimated returns due to omitted variables is likely offset by an equal downward bias resulting from measurement errors in reported schooling controlling for both of these potential sources of bias yields results comparable to conventional regression estimates of the economic return to schooling

dynamic replenishment production and pricing decisions in the face of supply disruption and random price sensitive demand abstract we study a joint decision problem for replenishment production pricing strategies in the face of both supply and demand uncertainties the supply of the raw material suffers from a potential supply disruption while the demand for the finished goods is price sensitive and random we assume that the raw material is storable and unfilled demands are fully backordered we characterize the optimal policies for raw material replenishment finished goods production and pricing we find that the optimal replenishment policy is an order up to type for a given finished goods stock level while it is a threshold type for a given raw material stock level for either a given finished goods stock level or a given raw material stock level the optimal production policy is a threshold type and the optimal pricing policy is a modified list price type respectively

continuous review inventory control when capacity is variable abstract many complex production inventory systems are characterized by uncertain capacities due to unexpected breakdowns unplanned repairs etc we analyze the effects of variable capacity on optimal lot sizing in continuous review environments two models are considered the basic eoq model and the order quantity reorder point model with backlogging for both cases we obtain the optimality conditions for generally distributed variable capacity we then show that the optimal order quantities for both models and the optimal reorder point for the second one are greater than those without variable capacity when the variable capacity distribution is exponential we develop effective procedures for finding the optimal solutions

inventory management with random supply and imperfect information a hidden markov model in most of the papers on inventory models operating in a random environment the state of the environment in each period is assumed to be fully observed with perfect information however this assumption is not realistic in most real life situations and we provide a remedy in this paper by assuming that the environment is only partially observed with imperfect information we accomplish this by analyzing two formulations of single item models with periodic review and random supply in a random environment in the first one supply is random due to random capacity of production and random availability of transportation we show that state dependent base stock policy is optimal if the capacity and all costs are observed while demand and availability are unobserved in the second model we consider a model with random availability only with fixed ordering cost we show that state dependent s s policy is optimal if the availability process is observable