Determining the Actual Time of a Variance Increment or Reduction Using the Moving Range Control Chart

ABSTRACT The monitoring of the variance of a process is important because it provides information about process improvement and deterioration. A process improvement is indicated by a reduction in the variance, whereas a process deterioration is shown by an increment in the variance. The R and S charts are widely used in the monitoring of the process variance for subgrouped data. In cases where the production rate is slow and monitoring of the process is required before the time needed to form a subgroup, control charting procedures based on individual measurements have been developed. This article discusses an approach to identify the exact time of a permanent shift in the process variance as a result of process improvement or deterioration using the maximum likelihood estimation techniques.