calculating order up to levels for products with intermittent demand abstract the croston 1972 forecasting and stock control for intermittent demands operational research quarterly 23 289 303 method is the standard method for forecasting intermittent demand it has been shown to perform well in various studies and is available in most commercial forecasting packages however little attention has been paid to linking the method with inventory control i e using the generated forecasts to calculate the inventory control parameters in this study we consider the most commonly applied order up to policy and show how the forecasts can be used to calculate order up to levels in a numerical study we show that the calculated order up to levels lead to service levels that are close to their targets

when do firing taxes matter abstract firing taxes have real effects only if layoffs are subsidized relative to quits as it is pareto optimal to label a separation a quit whenever doing so maximizes joint wealth given the size of layoff subsidies in oecd countries firing taxes are an unlikely candidate to explain cross country differences in labour market performance

natural groups and economic characteristics as driving forces of wage discrimination we investigate whether the origin of an employee provides different motives for wage discrimination in gift exchange experiments with students and migrant workers in china in a lab and an internet experiment subjects in the role of employers can condition their wages on the employees home provinces the resulting systematic differences in wages can be linked to natural groups and economic characteristics of the provinces in group favoritism increases wages for employees who share the same origin as the employer while an increased probability of being matched with an employee with a different ethnicity reduces wages furthermore wages in the laboratory increase with the actual wage level in the employees home province nevertheless employees effort is not influenced by these variables only the wage paid in the experiment influences effort

high school experiences the gender wage gap and the selection of occupation abstractusing within high school variation and controlling for a measure of cognitive ability this article finds that high school leadership experiences explain a significant portion of the residual gender wage gap and selection into management occupations our results imply that high school leadership could build non cognitive productive skills that are rewarded years later in the labour market and that explain a portion of the systematic difference in pay between men and women alternatively high school leadership could be a proxy variable for personality characteristics that differ between men and women and that drive higher pay and becoming a manager because high school leadership experiences are exogenous to direct labour market experiences our results leave less room for direct labour market discrimination as a driver of the gender wage gap and occupation selection

racial disparities in job finding and offered wages abstractthe extent to which discrimination can explain racial wage gaps is one of the most divisive issues in the social sciences using a newly available data set this paper develops a simple empirical test that under plausible but not innocuous conditions provides a lower bound on the extent of discrimination in the labor market taken at face value our estimates imply that differential treatment accounts for at least one third of the black white wage gap we argue that the patterns in our data are most naturally rationalized through a search matching model in which employers statistically discriminate on the basis of race when hiring unemployed workers but learn about their marginal product over time

evaluation of forecasting error measurements and techniques for intermittent demand abstract this paper is a part and contribution to develop flexible and robust supply chain forecasting systems under changing market demands it suggests new tools and models to evaluate forecasting error measurements the paper especially studies slow moving or intermittent demand when for items the forecasting time periods often have zero demand for the difficult to forecast intermittent demand the croston forecasting technique is mostly regarded as a better choice than single exponential smoothing these two methods croston and single exponential smoothing together with two modifications of the croston method are discussed and evaluated with real intermittent data the apprehended performance of a forecasting technique is dependent of the chosen measurement of forecast errors the main purpose is to examine and evaluate different forecasting error measurements traditional measurements of forecast errors are studied mean absolute deviation mad mean square error mse etc together with new suggested error and bias measurements periods in stock pis and number of shortages nos pis considers the time aspect of the forecast error nos considers the development of cumulated forecast error cfe a complementary measure for descriptive statistics of time series is also suggested mean average change the conclusion through principal components analysis pca is that a single measure of forecast errors does not present the total different dimensions of the errors complementary error measures must be used

on the stock control performance of intermittent demand estimators the purpose of this paper is to assess the empirical stock control performance of intermittent demand estimation procedures the forecasting methods considered are the simple moving average single exponential smoothing croston s method and a new method recently developed by the authors of this paper we first discuss the nature of the empirical demand data set 3000 stock keeping units and we specify the stock control model to be used for experimentation purposes performance measures are then selected to report customer service level and stock volume differences the out of sample empirical comparison results demonstrate the superior stock control performance of the new intermittent demand forecasting method and enable insights to be gained into the empirical utility of the other estimators

on the interaction between forecasting and stock control the case of non stationary demand the effect of using estimated forecast demand parameters on the performance of an inventory control system is an intriguing and important subject recent research has been undertaken on this phenomenon assuming stationary demand data in this paper we extend the research to non stationary demands by means of simulation the case of a periodic order up to level inventory system is considered and the experimental structure allows us to evaluate in a progressive manner the accumulated effect of using the optimal forecasting method optimal forecast parameters and correct variance expression procedures the results allow insights to be gained into operational issues and demonstrate the scope for improving stock control systems

intermittent demand forecasts with neural networks intermittent demand appears when demand events occur only sporadically typically such time series have few observations making intermittent demand forecasting challenging forecast errors can be costly in terms of unmet demand or obsolescent stock intermittent demand forecasting has been addressed using established forecasting methods including simple moving averages exponential smoothing and croston s method with its variants this study proposes a neural network nn methodology to forecast intermittent time series these nns are used to provide dynamic demand rate forecasts which do not assume constant demand rate in the future and can capture interactions between the non zero demand and the inter arrival rate of demand events overcoming the limitations of croston s method in order to mitigate the issue of limited fitting sample which is common in intermittent demand the proposed models use regularised training and median ensembles over multiple training initialisations to produce robust forecasts the nns are evaluated against established benchmarks using both forecasting accuracy and inventory metrics the findings of forecasting and inventory metrics are conflicting while nns achieved poor forecasting accuracy and bias all nn variants achieved higher service levels than the best performing croston s method variant without requiring analogous increases in stock holding volume therefore nns are found to be effective for intermittent demand applications this study provides further arguments and evidence against the use of conventional forecasting accuracy metrics to evaluate forecasting methods for intermittent demand concluding that attention to inventory metrics is desirable

analysis of order up to level inventory systems with compound poisson demand abstract we analyse a single echelon single item inventory system where the demand and the lead time are stochastic demand is modelled as a compound poisson process and the stock is controlled according to a continuous time order up to out level policy we propose a method for determining the optimal out level for cost oriented inventory systems where unfilled demands are backordered we first establish an analytical characterization of the optimal out level the actual calculation is based on a numerical procedure the accuracy of which can be set as highly as desired by means of a numerical investigation we show that the method is very efficient in calculating the optimal out level we compare our results with those obtained using an approximation proposed in the literature and we show that there is a significant difference in accuracy for slow moving items our work allows insights to be gained on stock control related issues for both fast and slow moving stock keeping units skus

assessing the effects of using demand parameters estimates in inventory control and improving the performance using a correction function inventory models need some specification of the distribution of demand in order to find the optimal order up to level or reorder point this distribution is unknown in real life and there are several solutions to overcome this problem one approach is to assume a distribution estimate its parameters and replace the unknown demand parameters by these estimates in the theoretically correct model earlier research suggests that this approach will lead to underperformance even if the true demand distribution is indeed the assumed one this paper directs the cause of the underperformance and quantifies it in case of normally distributed demand furthermore the formulae for the order up to levels are corrected analytically where possible and otherwise by use of simulation and linear regression simulation shows that these corrections improve the attained performance

adapting wright s modification of holt s method to forecasting intermittent demand abstract the challenge of accurately forecasting demand for spare parts is due to the intermittent nature of their demand this study compares a recent modification of croston s method and a method based on holt s double exponential smoothing taking the firm s competitive priorities into consideration wright s modification of holt s method is presented as a viable alternative when forecasting demand for spares especially when trend is present results indicate that firms focusing on minimizing inventory levels as a priority should consider forecasting using the modified croston s method if their priority is high customer service then the modified holt s method is superior

on the bias of intermittent demand estimates forecasting and inventory control for intermittent demand items has been a major problem in the manufacturing and supply environment croston operational research quarterly 23 1972 289 proposed a method according to which intermittent demand estimates can be built from constituent events croston s method has been reported to be a robust method but has shown more modest benefits in forecasting accuracy than expected in this research one of the causes of this unexpected performance has been identified as a first step towards improving croston s method certain limitations are identified in croston s approach and a correction in his derivation of the expected estimate of demand per time period is presented in addition a modification to his method that gives approximately unbiased demand per period estimates is introduced all the conclusions are confirmed by means of an extended simulation experiment where croston s and revised croston s methods are compared the forecasting accuracy comparison corresponds to a situation of an inventory control system employing a re order interval or product group review

an improved method for forecasting spare parts demand using extreme value theory inventory control for spare parts is essential for many organizations due to the trade off between preventing high holding cost and stockouts the lead time demand distribution plays a central role in inventory control the estimation of this distribution is problematic as the spare part demand is often intermittent and as a consequence often only a limited number of non zero data points are available in practice the well known empirical method uses historical demand data to construct the lead time demand distribution although it performs reasonably well when service requirements are relatively low it has difficulties in achieving high target service levels in this paper we improve the empirical method by applying extreme value theory to model the tail of the lead time demand distribution to make the most out of a limited number of demand observations we establish that extreme value theory can be applied to lead time demand periods computed over overlapping intervals we consider two service levels the expected waiting time and cycle service level our experiments show that our method improves the inventory performance compared to the empirical method and is competitive with the wss method croston s method and sba for a range of demand distributions

sex discrimination in restaurant hiring an audit study in an audit study of sex discrimination in hiring comparably matched pairs of men and women applied for jobs as waiters and waitresses at restaurants in philadelphia in high price restaurants where earnings are higher job applications from women had an estimated probability of receiving a job offer that was lower by about 0 4 and an estimated probability of receiving an interview that was lower by about 0 35 both estimated differentials are statistically significant additional evidence suggests that customer discrimination partly underlies the hiring discrimination

orchestrating impartiality the impact of blind auditions on female musicians a change in the audition procedures of symphony orchestras adoption of blind auditions with a screen to conceal the candidate s identity from the jury provides a test for sex biased hiring using data from actual auditions in an individual fixed effects framework we find that the screen increases the probability a woman will be advanced and hired although some of our estimates have large standard errors and there is one persistent effect in the opposite direction the weight of the evidence suggests that the blind audition procedure fostered impartiality in hiring and increased the proportion women in symphony orchestras

demand forecasting for multiple slow moving items with short requests history and unequal demand variance modeling the lead time demand for the multiple slow moving inventory items in the case when the available requests history is very short is a challenge for inventory management the classical forecasting technique which is based on the aggregation of the stock keeping units to overcome the mentioned historical data peculiarity is known to lead to very poor performance in many cases important for industrial applications an alternative approach to the demand forecasting for the considered problem is based on the bayesian paradigm when the initially developed population averaged demand probability distribution is modified for each item using its specific requests history this paper follows this approach and presents a new model which relies on the beta distribution as a prior for the request probability and allows to account for disparity in variance of demand between different stock keeping units to estimate the model parameters a special computationally effective technique based on the generalized method of moments is developed simulation results indicate the superiority of the proposed model over the known ones while the computational burden does not increase

the accuracy of a modified croston procedure a modified croston procedure to forecast intermittent demand is reviewed the modified procedure published in this journal in 2004 consists of a new method for estimating the mean demand and a smoothing method for estimating the variance of the forecasted demand rate it is found that the smoothed variance estimate is based on an invalid measure of forecast accuracy moreover it is shown that the method of forecasting mean demand produces biased forecasts the accuracy of this method is assessed using simulated data and the mean square error measure and compared with croston s method and single exponential smoothing it is found that croston s method is generally more accurate than its modification particularly for strongly intermittent series

do employer preferences contribute to sticky floors the authors investigate the importance of employer preferences in explaining sticky floors the pattern in which women are less likely as compared to men to start to climb the job ladder the authors perform a randomized field experiment in the belgian labor market and test whether hiring discrimination based on gender is heterogeneous by whether jobs imply a promotion compared to the applicants current position the findings show that women receive 33 fewer interview invitations when they apply for jobs that imply a first promotion at the functional level by contrast the results show that their hiring chances are not significantly affected by the authority level of the job

exploiting elapsed time for managing intermittent demand for spare parts we present an intermittent demand forecasting method that conditions on the elapsed time since the last demand occurrence to anticipate incoming demand and show using empirical data that this can substantially reduce both stock investment and lost revenue for spare parts management we extensively benchmark our method against existing forecasting and bootstrapping methods on forecast accuracy and inventory performance and demonstrate that its performance is robust under general conditions our method is the first to incorporate that activities at the demand side such as aggregation of demand preventive and corrective maintenance can lead to a positive relation between demand size and inter arrival time of demand occurrences by anticipating incoming demand our method offers substantial financial gains

lumpy demand forecasting using neural networks abstract the current study applies neural network nn modeling in forecasting lumpy demand it is to the best of our knowledge the first such study our study compares the performance of nn forecasts to those using three traditional time series methods single exponential smoothing croston s method and the syntetos boylan approximation we find nn models to generally perform better than the traditional methods using three different performance measures we also independently validate earlier findings that the syntetos boylan approximation performs better than the croston s and single exponential smoothing methods in lumpy demand forecasting

the effects of integrating management judgement into intermittent demand forecasts empirical research suggests that quantitatively derived forecasts are very frequently judgementally adjusted nevertheless little work has been conducted to evaluate the performance of these judgemental adjustments in a practical demand sales context in addition the relevant analysis does not distinguish between slow and fast moving items currently there are neither conceptual developments nor empirical evidence on the issue of integrating judgements and statistical forecasts for slow intermittent demand items moreover no results have ever been reported on the stock control implications of these human judgements our work analyses monthly intermittent demand forecasts for the uk branch of a major international pharmaceutical company the company relies upon a commercially available statistical forecasting system to produce forecasts that are subsequently judgementally adjusted based on marketing intelligence gathered by the company forecasters the benefits of the intervention are evaluated by comparing the actual sales to system and final forecasts using both forecast accuracy and inventory control accuracy implication metrics our study allows insights to be gained on potential improvements to intermittent demand forecasting processes and subsequently the design effectiveness of forecasting support systems

does equal pay legislation reduce labour market inequality this paper considers a labour market model of monopsonistic competition with taste based discrimination against minority workers to study the effect of equal pay legislation on labour market inequality when the taste for discrimination is small or competition is weak the policy removes job segregation and the wage gap completely however with a bigger taste for discrimination or stronger competition equal pay legislation leads to more job segregation and sometimes minority workers end up earning less than before profits of discriminating firms might increase and discrimination can persist in the long run although it would have disappeared without the policy

gender discrimination in job ads evidence from china we study explicit gender discrimination in a population of ads on a chinese internet job board gender targeted job ads are common favor women as often as men and are much less common in jobs requiring higher levels of skill employers relative preferences for female versus male workers on the other hand are more strongly related to the preferred age height and beauty of the worker than to job skill levels almost two thirds of the variation in advertised gender preferences occurs within firms and one third occurs within firmoccupation cells overall these patterns are not well explained by a firm level animus model by a glass ceiling model or by models in which broad occupational categories are consistently gendered across firms instead the patterns suggest a model in which firms have idiosyncratic preferences for particular job gender matches which are overridden in skilled positions by factors such as thinner labor markets or a greater incentive to search broadly for the most qualified candidate jel codes j16 j63 j71 copyright 2013 oxford university press

bargaining sorting and the gender wage gap quantifying the impact of firms on the relative pay of women there is growing evidence that rm specic pay premiums are an important source of wage inequality these premiums will contribute to the gender wage gap if women are less likely to work at high paying rms or if women negotiate worse wage bargains with their employers than men using longitudinal data on the hourly wages of portuguese workers matched with balance sheet information for rms we show that the wages of both men and women contain rm specic premiums that are strongly correlated with employer productivity we then show how the impact of these rm specic pay dierentials on the gender wage gap can be decomposed into a combination of bargaining and sorting eects consistent with the bargaining literature we nd that women receive only 90 of the rm specic pay premiums earned by men notably we obtain very similar estimates of the relative bargaining power ratio from our analysis of between rm wage premiums and from analyzing changes in

gender and job performance evidence from wall street this study concerns the relationship between gender and job performance among brokerage firm equity analysts women s representation in analyst positions dropped from 16 percent in 1995 to 14 percent in 2005 the study found significant gender based differences in performance on various dimensions for example women cover roughly 9 stocks on average as compared with 10 for men and women s earnings estimates tend to be less accurate than men s estimates but the study also found that women are significantly more likely than men to be designated as all stars which indicates that they outperform men in other aspects of job performance

control of inventories with intermittent demand abstract the problem of controlling inventories with intermittent demands is considered a method for determining re order levels consistent with a specified customer service level is proposed the distinguishing feature is the use of a probability distribution with a spike at zero to represent the relative frequency of periods with no transactions

forecasting and inventory performance in a two stage supply chain with arima 0 1 1 demand theory and empirical analysis the arima 0 1 1 demand model has been analysed extensively by researchers and used widely by forecasting practitioners due to its attractive theoretical properties and empirical evidence in its support however no empirical investigations have been conducted in the academic literature to analyse demand forecasting and inventory performance under such a demand model in this paper we consider a supply chain formed by a manufacturer and a retailer facing an arima 0 1 1 demand process the relationship between the forecasting accuracy and inventory performance is analysed along with an investigation on the potential benefits of forecast information sharing between the retailer and the manufacturer results are obtained analytically but also empirically by means of experimentation with the sales data related to 329 stock keeping units skus from a major european superstore our analysis contributes towards the development of the current state of knowledge in the areas of inventory forecasting and forecast information sharing and offers insights that should be valuable from the practitioner perspective