Resumen de Improved Shewhart-Type Charts for Monitoring Times Between Events
N. Kumar, S. Chakraborti, A. C. Rakitzis
In high-yield processes with a low defect rate, the recommendation is to monitor the times between events (TBE) with variables control charts in order to overcome the weaknesses of conventional attribute control charts. In this paper, we incorporate supplementary runs rules into one of the existing Shewhart-type TBE charts, namely the tr -chart, in order to improve its small to moderate size-shift detection ability. Moreover, because the existing tr -chart based on probability limits is biased, we provide a procedure for designing a tr -chart, with or without runs rules, that is unbiased. The performance of several tr -charts with runs rules is investigated, whereas comparisons with EWMA and CUSUM charts are also given. It is seen that some of the runs rules enhanced tr -charts perform substantially better. A real-data example is provided for illustration and some concluding remarks are given.
