Statistical Project of Control Chart with Variable Sample Size and Interval

Abstract

Control charts with adaptive schemes are tools used to monitor processes and to signal the presence of special causes. However, the use of adaptive schemes is not common yet because they are topics rarely covered in textbooks and are not available in traditional software used for statistical analysis. This work aims to present how to plan and estimate the optimal parameters of an adaptive chart for monitoring the mean of a process using sample size and variable interval ((X_bar-VSSI). The X_bar-VSSI chart was chosen because it is a scheme with great potential for practical application, for the chart only requires knowledge of the sample size and the time between sample selections after established the optimal parameters for the chart. Markov chains were used to evaluate the chart performance based on the average time between the instant when the process changes and the moment when the chart signals the condition out of control. It is presented two functions written in R language to assist the user in planning a statistical project based on the X_bar-VSSI adaptive scheme.

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