VIEWING MULTIPLE PROCESS STREAMS ON A BOX PLOT

The meaningful and correct application of control charts can be deceptively difficult. At first glance, introducing a control chart to the shop floor seems easy enough. Pick an important parameter to measure, determine a sampling strategy, teach someone how to plot the points and voila--a control chart is born. The results: audit scores are higher, Cpk values get reported and customers are happy to see statistical process control (SPC) activity. Using control charts might even result in cost savings. However, do the alarms generated by control charts provide justification to perform process interrogation? Are the sources of variation captured within each subgroup useful for identifying true process changes? These are just some of the questions that must be answered before the chart can be considered a successful process improvement tool.

Not to diminish the importance of passing audits and making customers happy, but a control chart's true function is to provide real-time feedback needed to control and improve processes. The enthusiasm enjoyed from the benefits listed above will fade if the data displayed on the charts do not help front line operators make better process decisions. Operators will reject SPC if they are required to feed data into a control chart that provides no useful information. Introducing useless statistical tools on the shop floor can negate improvement activities and jeopardize an SPC program's effectiveness.