An attribute control chart using discriminant limits for monitoring process under the Weibull distribution

A new efficient process monitoring scheme has been developed for a process under which the quality characteristic follows the Weibull distribution. Recently control charts using discriminant limits for process monitoring for a normal distribution have been developed. But, there are many situations in which the distribution of underlying data is not normal. So, the application of such chart to a skewed distribution like the Weibull distribution may lead to erroneous conclusions. In this paper, an attribute chart using discriminant limits for the Weibull distribution has been developed. The parameters of the proposed chart have been determined by considering the in-control average run lengths. An example using the simulation data has been included for the practical use of the proposed scheme. It has been observed that the proposed chart is efficient for the quick detection of an out-of-control process. A real example from a healthcare area is also added to show the application of the proposed control chart. © 2018, German Academic Society for Production Engineering (WGP).11Nscopu

O uso dos índices Cpk, Cpm e Cpmk para monitorar processos

The Cpk, Cpm and Cpmk indices was originally created to measure the ability of the processes to produce products meeting specifications but, more recently, they have also been used to control processes. In these new applications of the Cpk, Cpm and Cpmk indices, the specification limits are no longer limits that decide whether the item is conforming or nonconforming, they are tunning parameters that increase or decrease the speed with the control charts signal process changes in the mean and/or the variance. Therefore, this research proposes the use of control charts using the three process capability indices as an alternative to the traditional charts ̅ e R Through the Monte Carlo simulation, it was that the Cpk, Cpm and Cpmk control charts are great alternatives to the use of the joint ̅ and R charts, due they signal faster mean shifts, variance increases, and also combined mean shifts with variance increases.Os índices Cpk, Cpm e Cpmk foram originalmente criados para medir a capacidade dos processos de produzirem itens que atendam às especificações, mas, mais recentemente, eles também têm sido usados para controlar processos. Nessas novas aplicações dos índices Cpk, Cpm e Cpmk, os limites de especificação não são mais limites que decidem se o item está em conformidade ou não, são parâmetros de ajustes que aumentam ou diminuem a velocidade com que o gráfico de controle sinaliza mudanças na média e/ou na variabilidade do processo. Portanto, esta pesquisa propõe o emprego dos gráficos de controle utilizando os três índices de capacidade de processo como alternativa aos gráficos tradicionais de ̅ e R. Por meio da simulação Monte Carlo comprovou-se que os gráficos Cpk, Cpm e Cpmk são ótimas alternativas ao uso dos gráficos em conjunto ̅ e R, pois sinalizam, com maior rapidez, mudanças na média, aumentos na variância e as mudanças nas médias combinadas com aumentos na variância