RE: Trends up or down in Shewhart charts

[Steven_S_Prevette@rl.gov]

We have been using the following set of rules at Hanford with good success.
My experience is that usually a special cause can be found when one of these
trips:

     One point outside the control limits
     Two out of Three points two standard deviations above/below average
     Four out of Five points one standard deviation above/below average
     Seven points in a row all above/below average
     Ten out of Eleven points in a row all above/below average
     Seven points in a row all increasing/decreasing.

This list is a consolidation of several common sources, including Acheson
Duncan's Quality Control and Industrial Statistics.  At one time we tried
using a jump of 3 sigma from one point to the next, but there were too many
false alarms (no special cause could be found).  So it was struck from the
list

See also http:\\www.hanford.gov\safety\vpp\spc.htm

Steve Prevette
QA Engineer, ESH Radiological Compliance
Fluor Hanford, A Fluor Global Services Company
ASQ Certified Quality Engineer
steven_s_prevette@rl.gov
509-373-9371