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Abstract

This article describes a simulation to compare the effectiveness of Variable Sampling Intervals (VSI) with Fixed Sampling Intervals (FSI). This comparison is made using both techniques in conjunction with Shewhart X-bar charts and data that are unknowingly positively autocorrelated. Simulation was essential to providing these research results because to date no theoretical solution addresses the performance of the VSI Shewhart X-bar chart when the data are autocorrelated. Control limits, calculated from initial sample observations, are utilized. The use of VSI is found to cause a quicker time to signal for an out of control process. We conclude that while the use of variable sampling intervals is advantageous when a large shift in the mean is expected, the VSI procedure is more sensitive to the impact of autocorrelation.

Keywords

SPC quality control monitoring mean charts Shewhart

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A Simulation Study for Comparing Fixed with Variable Sampling Interval Shewhart X-Bar Control Charts in the Presence of Undetected Autocorrelated Data

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