Significance of rational sub grouping of data for Interpreting X bar and R Charts-A Case Study in a Service Organization
Keywords:
Average handling time, Control charts, Sub grouping, Variation, services, Six sigma.Abstract
Basic philosophy of Six Sigma methodology is to reduce variation in key product quality characteristics around specified target value. In that way, Statistical process control chart is a major constituent of a Six Sigma project as it provides a statistical test to determine whether average variation within a subgroup is consistent with variation between subgroups. Rational sub grouping plays an important role in the use of control charts (X bar and R charts). The subgroups are chosen so that variation within each subgroup is as small as feasible and there are utmost chances of process average to shift between subgroups. Incorrect sub grouping leads to useless control charts that do not detect the sources of variation in a process. The purpose of this article is to demonstrate the importance of rational sub grouping for identifying root causes of variation for a service process. The authors have used three different methods for sub grouping the data to plot X bar and R chart for Average Handling Time (AHT) taken by service executives to undertake various customer complaints and query processes.
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