International Journal of Quality Engineering and Technology (5 papers in press)

Regular Issues

• Steepest ascent improvement by response predictions implementation  
  by Vladimir B. Bokov 
  Abstract: For the exploratory runs of steepest ascent the use of response predictions is proposed. The factors for these runs are estimated on the data of initial experiment and by finding conditional extremum. Parameter estimates are employed to obtain estimated response functions on which the response of exploratory runs is predicted. Response prediction intervals for these runs are found by using the parameter estimates of linear models, factor estimates for exploratory runs, and linear models for response predictions. This allows to identify those response observations that go beyond the bounds of response prediction intervals and to find factor values under which exploratory run response at hand is still in its prediction interval. The comparison of two linear models reveals that the model with coded factors and response variables allows to obtain the smaller intervals of response predictions. This permits to identify more accurately the last run of steepest ascent and factor levels for design centre of the next initial experiment.
  Keywords: design of experiment; linear model; prediction interval.
  
  
• The Effect of Autocorrelation on Control Charts Performance and Process Capability Indices Calculation  
  by Martin Kovarik, Petr Bris 
  Abstract: Chemical processes as well as many non-industrial processes exhibit autocorrelation, for which the above-mentioned control procedures are not suitable. This paper considers the problem of monitoring a process in which the observations can be represented as a first-order autoregressive model following a heavy tailed distribution. It also presents practical usage of time series control charts on the chemical process of monitoring concentrations. The second part discusses the effect of autocorrelation on the process capability analysis when the observations are made by an autoregressive model of the first order. The process capability indices provide a measure of how a process fits within the specification limits. When calculating indices, it is usual to assume that the process data are independent. The simulation study in this part discusses the effect of a higher autocorrelation order on process capability indices.
  Keywords: statistical process control; autocorrelation; control charts; process capability indices; process variability; time series modelling.
  
  
• Optimisation of two-dimensional warranty region under preventive maintenance over product lifetime by considering both manufacture and consumers point of views  
  by Ali Salmasnia, Maryam Baratian, Mehdi Ghazanfari, Hossein Fallah-Ghadi 
  Abstract: An appropriate warranty period plays an essential role in consumer satisfaction and manufacturer costs. Manufacturer and consumer are two critical parts of any transaction; therefore, inattention to each of them causes dissatisfaction and financial loss. Therefore, this research proposes a model to determine an optimal two-dimensional warranty period to minimise both manufacturer's warranty servicing cost and consumers post-warranty cost. Furthermore, to reduce the frequency of product failures, a preventive maintenance program is implemented during the warranty and post-warranty periods, respectively, by the manufacturer and the consumer. Finally, two comparisons are made to show the model effectiveness. The first one shows the superiority of the joint minimisation of the manufacturer's warranty cost and the consumer's post-warranty cost over the single minimisation. The second one shows that a proper PM program can effectively reduce the costs of both the manufacturer and the consumer.
  Keywords: two-dimensional warranty; periodic preventive actions; manufacturer’s point of view consumer’s viewpoint.
  
  
• Measurement System Analysis of Viscosity: a Six-Sigma Gage Repeatability and Reproducibility Analysis  
  by Doraid Dalalah 
  Abstract: Different viscometer techniques are used to measure the viscosity; however, the repeatability and reproducibility of these measurement systems have not been investigated. Literature suggests that viscometers exhibit measurement variations that can be categorized into variations due to the process, the gage, and the operators. To explore the sources of variations in viscometers, a Six-Sigma gage repeatability and reproducibility (GR&R) study is conducted. GR&R studies utilize two-way ANOVA to separate measurement variation into different components to find the precision-to-tolerance, signal-to-noise ratios and the true process capability index. A software is built to demonstrate the newly introduced measurement assessment chart that pinpoints the suggested but necessary corrective actions. The results of a real industrial data show that rotational viscometers are more reliable than orifice viscometers, however, with moderate capability due to their modest signal-to-noise ratio. The model, assessment chart and the software can be used to assess other gages.
  Keywords: Gage; Repeatability; Reproducibility; Errors; Signal-to-noise ratio; Precision-to-tolerance ratio; Viscosity.
  
  
• A NEW PROBABILITY BASED MULTIVARIATE PROCESS CAPABILITY INDEX  
  by Kailas Khadse, Ramkrishna Shinde 
  Abstract: Multivariate process capability indices (MPCIs) MCp, MCpk and MCpm have been proposed in literature to measure multivariate process capability. From practitioners view, the intention of this paper is to propose a new probability based MPCI MSpmk which takes into account process variability, departure of the process mean from target and proportion of conformity when assessing multivariate process performance. The MSpmk is the extension in multivariate setup of univariate process capability index Spmk. The Spmk is smooth version of widely used univariate process capability index Cpmk. The MSpmk is very easy to compute and combines the merits of MPCIs MCp, MCpk and MCpm. Proposed MPCI assumes that production process is multivariate normal and specification region is rectangular. Proposed MPCI is recommendable with respect to its performance in various scenarios and its natural estimator statistical properties for real world usage.
  Keywords: probability based multivariate process capability index; multivariate normal process; rectangular specification region; proportion of conformity; multivariate process performance; measures of multivariate process capability.