Bounding Optimal Expected Revenues for Assortment Optimization under Mixtures of Multinomial Logits

We consider assortment problems under a mixture of multinomial logit models. There is a fixed revenue associated with each product. There are multiple customer types. Customers of different types choose according to different multinomial logit models whose parameters depend on the type of the customer. The goal is to find a set of products to offer so as to maximize the expected revenue obtained over all customer types. This assortment problem under the multinomial logit model with multiple customer types is NP‐complete. Although there are heuristics to find good assortments, it is difficult to verify the optimality gap of the heuristics. In this study, motivated by the difficulty of finding optimal solutions and verifying the optimality gap of heuristics, we develop an approach to construct an upper bound on the optimal expected revenue. Our approach can quickly provide upper bounds and these upper bounds can be quite tight. In our computational experiments, over a large set of randomly generated problem instances, the upper bounds provided by our approach deviate from the optimal expected revenues by 0.15% on average and by less than one percent in the worst case. By using our upper bounds, we are able to verify the optimality gaps of a greedy heuristic accurately, even when optimal solutions are not available.     

Visceral adiposity index is associated with premature ejaculation inversely: a cross-sectional study

Objective: Visceral adiposity index (VAI) is a novel indicator for the assessment of visceral obesity. In this study, we aimed to evaluate the relationship between VAI and premature ejaculation (PE).

Materials and method: A total of 300 men were included in the study. Hundred and fifty men with PE and 150 men without PE (control). All men were evaluated for PE by premature ejaculation diagnostic tool (PEDT). VAI levels were calculated using body mass index (BMI), high density lipoprotein and triglyceride (TG) levels.

Results: Mean age of the study groups was 34.3?±?5.2 (30–60) years and the mean age of the controls were 35.9?±?5.3 (30–60) years. The men with PE had lower BMI, TG levels, waist circumference (WC) and higher high-density lipoprotein-cholesterol (HDL-C) levels. Mean VAI level was 4.13?±?0.7 in study group and 5.72?±?1.6 in control group, respectively. VAI levels were statistically higher in men without PE (p?Discussion: Our cross-sectional study demonstrated a negative correlation between VAI and PE. VAI is superior index for the evaluation and calculation the relationship between obesity and PE.     

A new generalization of skew-T distribution with volatility models

In this paper, we propose a new generalized alpha-skew-T (GAST) distribution for generalized autoregressive conditional heteroskedasticity (GARCH) models in modelling daily Value-at-Risk (VaR). Some mathematical properties of the proposed distribution are derived including density function, moments and stochastic representation. The maximum likelihood estimation method is discussed to estimate parameters via a simulation study. Then, the real data application on S&P-500 index is performed to investigate the performance of GARCH models specified under GAST innovation distribution with respect to normal, Student's-t and Skew-T models in terms of the VaR accuracy. Backtesting methodology is used to compare the out-of-sample performance of the VaR models. The results show that GARCH models with GAST innovation distribution outperforms among others and generates the most conservative VaR forecasts for all confidence levels and for both long and short positions.     

Value of and Interaction between Production Postponement and Information Sharing Strategies for Supply Chain Firms

We analyze the value of and interaction between production postponement and information sharing, which are two distinct strategies to reduce manufacturers’ uncertainty about demand. In both single‐level and two‐level supply chains, from the manufacturer's perspective, while information sharing is always valuable, production postponement can sometimes be detrimental. Furthermore, the value of production postponement is not merely driven by savings in inventory holding cost as postponement enables the manufacturer to avoid both excess and shortfall in production. We find that production postponement and information sharing strategies may substitute, complement, or conflict with each other, depending on the extent of the increase in the unit production cost when production is postponed. In a two‐level supply chain, from the retailer's perspective, information sharing and production postponement can be beneficial or detrimental. When information sharing is beneficial to the retailer, the retailer always shares her demand information with the manufacturer voluntarily. In addition, this voluntary information sharing is truthful because inflated or deflated demand information hurts the retailer through a higher wholesale price or a stock‐out. However, the retailer never shares her demand information voluntarily if the manufacturer has already adopted production postponement because production postponement and information sharing strategies always conflict with each other. Even when the retailer does not benefit from information sharing, we show that the manufacturer can always design an incentive mechanism to induce the retailer to share the demand information, irrespective of whether the manufacturer has already implemented production postponement or not. The above findings underscore the need for a careful assessment of demand uncertainty‐reduction strategies before the supply chain players embark upon them.     

Balancing Revenues and Repair Costs under Partial Information about Product Reliability

We consider the problem faced by a company selling a product with warranty and under partial information about the product reliability. The product can fail from multiple failure types, each of which is associated with an inherently different repair cost. If the product fails within the warranty duration, then the company is required to pay the repair cost. The company does not know the probabilities associated with different failure types, but it learns the failure probabilities as sales occur and failure information is accumulated. If the failure probabilities turn out to be too high and it becomes costly to fulfill the warranty coverage, then the company may decide to stop selling the product, possibly replacing it with a more reliable alternative. The objective is to decide if and when to stop. By formulating the problem as a dynamic program with Bayesian learning, we establish structural properties of the optimal policy. Since computing the optimal policy is intractable due to the high dimensional state space, we propose two approximation methods. The first method is based on decomposing the problem by failure types and it provides upper bounds on the value functions. The second method provides lower bounds on the value functions and it is based on a deterministic approximation. Computational experiments indicate that the policy from the first method provides noticeable benefits, especially when it is difficult to form good estimates of the failure probabilities quickly.     

Visceral adiposity index is associated with benign prostatic enlargement in non-diabetic patients: a cross-sectional study

Objective: To evaluate the association between visceral adiposity index (VAI) - a novel indicator for the assessment of visceral adipose tissue and prostate enlargement in non diabetic patients.

Material and methods: Four hundred patients who were admitted to the Urology clinic between January and December 2014 with complaints of BPH(benign prostatic hyperplasia )/LUTS(male lower urinary tract symptoms)were enrolled in this cross-sectional study. Patients were divided into two groups according to their prostate volume and international prostate symptom score (IPSS) value. They were compared in terms of age, body mass index (BMI), VAI, prostate volume, PSA, post micturional residual volume (PMRV), uroflowmetry Q max value, triglyceride (TG), high density lipoprotein-cholesterol (HDL-C) and fasting blood sugar (FBS).

Results: Although univariate analyses reveal that age, BMI, waist circumference (WC), FBS, TG, HDL-C level and TG/HDL ratio were correlated with prostate volume, only age [1.125 OR (1.088–1.164), p?=?.00001], BMI [1.119 OR (1.040–1.204), p?=?.003], TG [1.043 OR (1.016–1.071), p?=?.002], HDL-C [0.923 OR (0.860–0.990), p?=?.025] and VAI [1.194 OR (1.110–1.305), p?=?.011] were statistically significant in multivariate analysis. A positive correlation was found between VAI value and prostate volume in the Spearman correlation test (r?=?0.29, p?=?.00001). The calculated area under the curve (AUC) for prostate volumes of 30, 40 and 50?ml were 0.680 (0.621–0.738), 0.625 (0.570–0.681) and 0.590 (0.528–0.652), respectively.

Conclusion: Our study revealed a positive correlation between VAI and prostate volume. Our results are needed to be tested with well-designed randomized prospective cohort studies.     

Increased visceral adiposity index associated with sexual dysfunction in men

Objective: Visceral adipose index (VAI) is a novel parameter for the evaluation of visceral obesity. As we know that obesity is a risk factor for erectile dysfunction (ED). So, in this study, we compared the VAI levels between the men with ED and without ED.

Materials and method: A total of 177 men were included in the study. Ninety-five men with ED and 82 men without ED (control). All men were evaluated for ED by Index of Erectile Function-5 items (IIEF-5). VAI levels were calculated using body mass index, high density lipoprotein and tryglyceride levels.

Results: Mean age was 53.5 (38–69) in men who have ED and 53.1 (34–69) in control. The men with ED had higher body mass index (BMI), triglyceride (TG) levels, higher waist circumference (WC) and lower high-density lipoprotein-cholesterol (HDL-C) levels. Mean VAI level was 5.18?±?2.50 in study group and 3.47?±?1.76 in control goup, respectively. VAI levels were statistically higher in men with ED (p?Discussion: The simplicity of WC and BMI measurement and TG and HDL assessment, make VAI an easily applicable index for the evaluation of visceral fat dysfunction. VAI can be useful index for the evaluation and calculation of erectile dysfunction risk.     

The concept of horizontal hierarchy and the organization of interorganizational networks: A comparative analysis

This paper is an attempt to describe the organization of interorganizational fields with the concept of horizontal hierarchy. It specifies certain structural properties of interorganizational field based on different types of linkages between organizations, and develops testable hypotheses by focusing on the interrelationships between the properties of these linkages within the conceptual definition of horizontal hierarchy. These hypotheses are later tested on data collected from manpower organizations in 17 communities of a large midwestern state.     

The association between triglyceride high density lipoprotein cholesterol ratio and benign prostate hyperplasia in non-diabetic patients:a cross-sectional study

Objectives: To assess the association between triglyceride (TG)/high density lipoprotein (HDL) ratio and benign prostate hyperplasia/lower urinary tract symptoms (BPH/LUTS).

Methods: Four hundred patients who were admitted to the Urology Clinic between January and December 2014 with complaints of BPH/LUTS were enrolled in this cross-sectional study. Patients were divided into two groups according to their International Prostate Symptom Score and prostate volume (PV). They were compared in terms of age, body mass index (BMI), PV, PSA, post micturional residual volume, uroflowmetry Q max value, fasting blood sugar, TG and high density lipoprotein-cholesterol (HDL-C) level and TG/HDL ratio.

Results: Although univariate analyses reveal that age, BMI, waist circumference (WC), FBS, TG, HDL-C level, and TG/HDL ratio were correlated with PV, only age [1.125 OR (1.088–1.164), p?=?.00001], BMI [1.119 OR (1.040–1.204), p?=?.003], TG [(1.043 OR (1.016–1.071), p?=?.002], HDL-C [(0.923 OR (0.860–0.990), p?=?.025], and TG/HDL ratio [(1.224 OR (1.130–1.315), p?=?.014] were statistically significant in multivariate analysis. The calculated area under the curve (AUC) for PV of 30?ml, 40?ml, and 50?ml was 0.668 (0.608–0.727), 0.617 (0.561–0.673), and 0.592 (0.530–0.654), respectively.

Conclusions: Our results indicate that the TG/HDL ratio correlates with enhancement in PV. Further studies are warranted to better evaluate this relationship.     

A New Dynamic Programming Decomposition Method for the Network Revenue Management Problem with Customer Choice Behavior

In this paper, we propose a new dynamic programming decomposition method for the network revenue management problem with customer choice behavior. The fundamental idea behind our dynamic programming decomposition method is to allocate the revenue associated with an itinerary among the different flight legs and to solve a single‐leg revenue management problem for each flight leg in the airline network. The novel aspect of our approach is that it chooses the revenue allocations by solving an auxiliary optimization problem that takes the probabilistic nature of the customer choices into consideration. We compare our approach with two standard benchmark methods. The first benchmark method uses a deterministic linear programming formulation. The second benchmark method is a dynamic programming decomposition idea that is similar to our approach, but it chooses the revenue allocations in an ad hoc manner. We establish that our approach provides an upper bound on the optimal total expected revenue, and this upper bound is tighter than the ones obtained by the two benchmark methods. Computational experiments indicate that our approach provides significant improvements over the performances of the benchmark methods.