Yiannis Nikolaidis

Comparing the Effectiveness of Various Bayesian X Control Charts

In an attempt to improve the procedures for statistical process control many researchers have developed and proposed a variety of adaptive control charts in the last decade. The common characteristic of those charts is that one or more of the chart parameters (sampling interval, sample size,control limits) is allowed to change during operation, taking into account current sample information. Due to their flexibility, adaptive charts are more effective than their static counterparts but they are also more complex in terms of implementation. The purpose of this paper is to evaluate the economic performance of various adaptive control schemes to derive conclusions about their relative effectiveness. The analysis concentrates on Bayesian control charts used for monitoring the process mean in finite production runs. We present dynamic programming formulations and properties of the optimal solutions, which we then use to solve a number of numerical examples. The results from our comparative numerical study indicate that the chart parameter having the most positive impact on the economic performance by being adaptive is the sampling interval. It is therefore sufficient in most cases to use control charts with adaptive sampling intervals rather than other types of partially adaptive charts or the more complicated fully adaptive control charts.

A multi-period model for managing used product returns

During the last decades there has been a consistent need for companies to manufacture ‘green’ products in order to contribute to environmental protection. The utilisation of used products (literally, the extension of their useful life cycle) is an excellent, indirect way for companies to conform to this requirement and, at the same time, increase their profit. In this paper a mixed integer linear programming mathematical model is proposed, which can be used for the optimisation of procurement, remanufacturing, stocking and salvaging decisions. The model is flexible enough to incorporate multiple suppliers, several quality levels of returned products and multiple periods of time. The applicability of the model is demonstrated through the optimisation of alternative scenarios, the optimal solutions of which reveal the potential profitability of used products exploitation.

A modelling framework for the acquisition and remanufacturing of used products

In recent times, there has been a consistent need for companies to produce ‘green’ products and offer ‘green’ services in order to contribute to environmental protection. The utilisation of used devices (extending their useful life cycle) is an excellent, indirect way for companies to conform to this requirement and, at the same time, increase their profit. Cell phones constitute one of the most interesting cases of products, which can be returned, remanufactured and reused: their replacement rate is large, the available quantity for reuse is huge and, consequently, the profit potential is significant. Motivated by the real case of a company involved in the acquisition and remanufacturing of used cell phones, a simple mathematical programming model is proposed in this work that can help remanufacturing companies to make optimal decisions concerning the quantities to be purchased and remanufactured. Its use, namely the simulation of the model stochastic parameters and the optimisation of the model, reveals not only that the exploitation of used products can be profitable, but also that as the ‘product acquisition system’ improves, the economic benefits for any remanufacturing company can be even greater.

Using economically designed Shewhart and adaptive ―X charts for monitoring the quality of tiles

This paper presents the economic design of ―X control charts for monitoring a critical stage of the main production process at a tile manufacturer in Greece. Two types of ―X charts were developed: a Shewhart-type chart with fixed parameters and adaptive charts with variable sampling intervals and/or sample size. Our prime motivation was to improve the statistical control scheme employed for monitoring an important quality characteristic of the process with the objective of minimizing the relevant costs. At the same time we tested and confirmed the applicability of the theoretical models supporting the economic design of control charts with fixed and variable parameters in a practical situation. We also evaluated the economic benefits of moving from the broadly used static charts to the application of the more flexible and effective adaptive control charts. The main result of our study is that, by redesigning the currently employed Shewhart chart using economic criteria, the quality-related cost is expected to decrease by approximately 50% without increasing the implementation complexity. Monitoring the process by means of an adaptive ―X chart with variable sampling intervals will increase the expected cost savings by about 10% compared with the economically designed Shewhart chart at the expense of some implementation difficulty. Copyright

On the student evaluation of university courses and faculty members’ teaching performance

Trying to determine higher education quality, one gets quickly to one of its significant dimensions, namely the quality of faculty members’ teaching. The latter and, overall, the quality of any university course should be certainly evaluated by their recipients, namely students. In this paper we develop a statistical framework based on Statistical Quality Control mainly, which can be used in order to exploit student evaluations as much as possible. More specifically we present two directions of data monitoring and analysis; the one uses control charts and the other hypotheses testing. The results that can be raised through both directions are crucial for any decision maker.

Economic Evaluation of ISO 2859 Acceptance Sampling Plans Used with Rectifying Inspection of Rejected Lots

ABSTRACT To conduct acceptance sampling, companies often use plans that are determined by easy-to-use standards. However, these standards do not take quality costs directly into account. Motivated by the case of a Greek company, which uses the Greek equivalent to the ISO 2859 (1974) for the quality control of its incoming raw materials, this article aims to evaluate the single-sampling plans recommended by the latest update of ISO 2859 from an economic point of view. The evaluation shows that the use of standards rarely leads to satisfactory economic results. Therefore, simple rules for the economical use of the standard are provided.

A more accurate formulation for a class of models for the economic design of control charts

Some of the published models for the economic design of control charts use the expected cost per unit of output as the objective function to be minimized. In these models, the computation of steady-state probabilities that the process is in each possible state does not take into account the effect of the corrective action that may follow a signal from the control chart. As a result, the expected cost per unit is overestimated and the selection of the chart parameters is not optimal. The purpose of this paper is to eliminate this inaccuracy by proposing the exact formulation and to estimate the magnitude of errors resulting from the inaccurate formulation of the objective function. By solving numerical examples of joint design of

New indices for the evaluation of the statistical properties of Bayesian x¯ control charts for short runs

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Reverse Logistics and QualityManagement Issues: State-of-the-Art

and R charts, it is shown that these errors are typically very large and consequently it is imperative to use the exact formulation, proposed in this paper, to avoid inefficient and costly control chart designs. Finally, an additional opportunity for further cost improvements in process control is identified and discussed.

Education on Corporate Social Responsibility in Greece: Is There Any and Does it Affect Students’ Perception?

In this paper we evaluate the statistical performance of various adaptive control schemes, which are implemented for monitoring the process mean in finite production runs. We develop formulas for the computation of a variety of indices, which reflect the statistical properties of the quality control mechanism of a production process, which is conducted through several one-sided, Bayesian x¯ control charts. Moreover, through our numerical study we evaluate the statistical performance of the economically designed adaptive x¯ control charts, in comparison with their static counterpart. Surprisingly, we find out that very often Bayes-nx¯ chart – with adaptive sample sizes but constant sampling intervals – performs better than the other Bayesian control charts, from a statistical point of view.