Dynamic Variation Changes

Dr Juergen Ude Dr Juergen Ude

Although step changes in the standard deviation are the least popular, they should be of equal importance to other charts and in some instances even more important. In manufacturing standard deviation affects process capability and thus non-conformance. For health the average blood pressure alone is insufficient to make medication decisions. A sudden increase in standard deviation can indicate extreme fluctuation in stress levels and may require alternative treatment to medication.

Increases in standard deviations can be reflected in increases of the mean, if lower values are bounded. For weights and measures control, depending on the legislation, increases in standard deviation due to a faulty filling process will require increasing ‘give-away’ to ensure weights and measures legislation is met.

Shewhart chart demonstrating the issues with spc theory Change Analysis - Step Changes in the Standard Deviation

Standard deviations for Sd SPC charts are the subgroup standard deviations if sub groups are greater than one. If the subgroups are equal to one, moving standard deviations are computed. For Change Analysis of Standard Deviation, the calculations are similar, but for subgroup size equal to one requires first removing changes in the process mean, as these would inflate the moving standard deviations.

There is no formula to perform change analysis. Instead statistically based machine powered algorithms are used.

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