Column: Technical Aids: Monitoring Reduction in Variation with a Range Chart

Modern practice in process studies focuses on reducing variation. The objective of this piece is to underscore the importance of this idea and to suggest how Shewhart control charts for the range can be modified to enhance the detection of a decrease in variation. Examination of the stability and magnitude of variation in all processes where control followed by improvement is important had its formal origin in Shewhart's (1931) seminal work. Shewhart argued that the standard deviation (s.d.) was to be preferred over the range for monitoring variation. The two main reasons he gave for this were: 1) the s.d. is more efficient, and 2) the s.d. is less dependent on the form of the parent distribution