Andrés Carrión

Statistical properties of the lsi multivariate control chart

Recently there has been a growing interest in multivariate statistical process control in which the simultaneous control of several related quality characteristics is considered The detection of a shift in the in-controi mean vector has already been studied in depth assuming that the p variables to be controlled are distributed as a p-multivariate normal. However, very little attention has been paid to the detection of shifts in the dispersion of the variables, i. e., shifts in the in-control covariance matrix ∑,0. The use of a chart which consists of plotting values of the generalized variance, |S|, is recommended in the bibliography, but its statistical properties are not well known In this paper we shall study the distribution of |S| and we shall show how to obtain a suitable control limit. The power of the chart wiii be studied and we shall show that it only depends on |∑1|/|∑0| where ∑1 is the out-of-control covariance matrix.

Comparing risk of failure models in water supply networks using ROC curves

Abstract The problem of predicting the failure of water mains has been considered from different perspectives and using several methodologies in engineering literature. Nowadays, it is important to be able to accurately calculate the failure probabilities of pipes over time, since water company profits and service quality for citizens depend on pipe survival; forecasting pipe failures could have important economic and social implications. Quantitative tools (such as managerial or statistical indicators and reliable databases) are required in order to assess the current and future state of networks. Companies managing these networks are trying to establish models for evaluating the risk of failure in order to develop a proactive approach to the renewal process, instead of using traditional reactive pipe substitution schemes. The main objective of this paper is to compare models for evaluating the risk of failure in water supply networks. Using real data from a water supply company, this study has identified which network characteristics affect the risk of failure and which models better fit data to predict service breakdown. The comparison using the receiver operating characteristics (ROC) graph leads us to the conclusion that the best model is a generalized linear model. Also, we propose a procedure that can be applied to a pipe failure database, allowing the most appropriate decision rule to be chosen.

The variable sample size variable dimension T2 control chart

In this paper, we describe the development of the variable dimension and variable sample size T2 control chart (VSSVDT2), which is an enhancement of the variable dimension T2 chart (VDT2). In the VDT2 control chart, the number of variables that are measured to compute the T2 statistic is made variable. Some of the variables are easy or inexpensive to measure and are always monitored. The variables that are more difficult or expensive to measure are measured only when the T2 value from the previous sample exceeds a specified threshold. The VDT2 control chart performs well for moderate and large shifts in the mean vector. However, its performance for small shifts is poor. To improve the chart’s performance in detecting such shifts, we propose the application of the variable sample size technique to the VDT2 control chart, resulting in the VSSVDT2 control chart. To promote the use of the VSSVDT2 chart, a user-friendly software has been developed, which the final user can use to find the best parameters of the VSSVDT2 chart for a particular process.

Modelling the failure risk for water supply networks with interval-censored data

Abstract In reliability, sometimes some failures are not observed at the exact moment of the occurrence. In that case it can be more convenient to approximate them by a time interval. In this study, we have used a generalized non-linear model developed for interval-censored data to treat the life time of a pipe from its time of installation until its failure. The aim of this analysis was to identify those network characteristics that may affect the risk of failure and we make an exhaustive validation of this analysis. The results indicated that certain characteristics of the network negatively affected the risk of failure of the pipe: an increase in the length and pressure of the pipes, a small diameter, some materials used in the manufacture of pipes and the traffic on the street where the pipes are located. Once the model has been correctly fitted to our data, we also provided simple tables that will allow companies to easily calculate the pipe׳s probability of failure in a future.

Optimal Design of Pre‐control Plans

Pre-control is a quality tool for quick set-up approvals, especially used in short-run processes. It is based on specifications instead of the natural variability of the process and uses cumulative counts in order to yield a conclusion. Its main drawbacks are a high false alarm rate and a low power to detect process deviations, under certain circumstances. These issues can be addressed by making the technique more flexible, as shown in previous works. In this paper, we introduce a Mathematical Programming approach in order to optimally determine the value of the pre-control parameters, so that it can meet the users requirements while minimizing the sample size of the technique as much as possible. We propose and develop a mathematical model for optimal pre-control and perform some numerical experiments in order to show its effectiveness. Copyright

Diseño de un Cuestionario para Valorar los Atributos de Grupos de Interés Universitarios desde un Enfoque de Liderazgo Participativo

In this paper the constructs of a proposed questionnaire that assess the university stakeholders attributes (power, legitimacy and urgency) are validated. From a subjective perspective and a relational approach, the perceptions of a sample of public universities leaders from Manizales (Colombia) were collected. Two groups of constructs were formed, the first includes the university stakeholders attributes mentioned above and the second collects their relevance. The Cronbach alpha values indicate that is feasible to measure effectively the proposed constructs. It is concluded that, from an individual level of analysis, the proposed questionnaire has the internal consistency and the measure reliability for assessing the university stakeholders attributes. In this analysis, it is considered that the organizational context determines its stakeholders and that leaders´ perceptions determine the relevance of the attributes.

Artificial neural networks for identifying the signals of multivariate EWMA control charts

Multivariate quality control charts show some advantages to monitor several variables in comparison with the simultaneous use of univariate charts, nevertheless, there are some disadvantages. The main problem is how to interpret the out-of-control signal of a multivariate chart. The MEWMA quality control chart is a very powerful scheme to detect small shifts in the mean vector. There are no previous specific works about the interpretation of the out-of-control signal of this chart. In this paper neural networks are designed to interpret the out-of-control signal of the MEWMA chart, and the percentage of correct classifications is studied for different cases.

The characteristics matrix as a tool for analysing process structure

The characteristics matrix is a tool to describe the relationship between product characteristics and process operations. It has been used traditionally with only descriptive purposes and analysed with a very limited intuitive approach. In this paper we present a methodology to analyse deeply the process structure using the information contained in the characteristics matrix. We apply the principles of the mic-mac method to study both direct and indirect internal relations, defining importance criteria and identifying the most important characteristics and operations.