Polyhedrika

  Statistical Process Control (SPC) is used to control a critical process output, acting on the factors affecting it, only when required. The purpose is to differentiate between a statistically significant change in the process, and common cause variation, because the actions required will be different. Process stability is a prerequisite for any further analysis. If a process is not stable, any conclusions you draw, can't be assumed to hold in the future. With SPC we check for process stability in the future. Control over-reaction A case of over-reaction is illustrated by the following example: Someone is shooting at a target, and based on the deviation of the impacts, he adjusts the gun site after each shot. The result will be an increase of the dispersion of the impacts; therefore, the adjustments will make the process worse. The correct way is, of course, to fire 5 or 6 shots without adjustments, and then decide if adjustment is required, based on the center of the impacts. ...

  The first step for process improvement is stability. We analyse this with Statistical Process Control . As a result of this analysis we may find out-of-control situations due to special causes. We need to resolve these causes to make the process stable.  Stability means the process is behaving in a predictable way but it doesn't necessarily mean it meets the customer expectations.  To find out to what extent is the process able to meet the customer expectations we measure Process Capability . Descriptive Statistics with Excel Download a copy of Excel file   Capability.xlsx   from OneDrive on to your PC to run it. In sheet Diameter  we have placed in column A our data consisting of 500 measurements of a critical diameter. After being confident that our process is stable we can characterize it with descriptive statistics in Excel Data Analysis:   These are the results: basic statistics of our data  Frequency Distribution Histogram The histo...