Mézquita-Fabián, Orlando J.

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    Diseño de gráfico EWMA para la dispersión del proceso usando la transformación de la normal inversa
    (2011) Mézquita-Fabián, Orlando J.; Artiles-León, Noel; College of Engineering; Carlo, Héctor J.; Ferrer-Alameda, Mercedes; Department of Industrial Engineering; Amador Dumais, Maria
    In this thesis two control charts for monitoring the dispersion of a variable are designed and compared. Both charts are based on an Exponentially Weighted Moving Average (EWMA) and the inverse normal transformation is used to improve the ability of the charts for detecting a change in the variability of the variable that is being monitored.A control chart with two control limits was introduced to monitor increases and decreases in the dispersion of the process and a control chart with one control limit was introduced to monitor increases in process dispersion. The Average Run Length (ARL) of the charts was obtained using Markov Chains and the design of the charts was made possible through regression functions that provide optimal values for parameters λ and K. When comparing the charts, we determined that the control chart with two control limits detects changes in process dispersion faster than the EWMA chart based on a logarithmic transformation, and faster than the control charts R and S for less than 50 percent increase in process dispersion. For the case of the control chart with one control limit, the performance was similar to the control chart based on a logarithmic transformation introduced by Crowder and Hamilton (1992) but have a better performance than charts R and S.