Six Sigma SPC OEM Products
Six Sigma and SPC (Statistical Process Control) Software
Six Sigma SPC - 2070 W. Washington St. #5
Springfield IL 62702 Ph: 217.698.0063
6 Sigma Statistical Process Control (SPC) Software for Windows
Since a process capability is null
and void unless the process is stable and in control, always look at
the control charts,
(X Bar and R
Bar), first. Once statistical control is established, then the
capability may be analyzed properly. What is considered ‘stable
and in control’ becomes another issue.
If your data is from an even and
symmetrical distribution, such as the proverbial ‘Normal
Distribution’, the data will be evenly distributed around the center
of the mean. If the data is not roughly evenly distributed around the
center or mean of the Histogram, it is called ‘skewed’. If it appears
skewed, you should understand the cause of the ‘skewness’.
If several peaks occur in the
distribution, look for the possibility that the data is coming from
two different sources, such as two separate production lines, or two
Compare the abnormal and normal
indices. Capability indices are quite sensitive to assumptions of the
A Capability index is a statistic,
subject to statistical error.
Most Engineers or Black
Belts consider a capable process to be one that has a Cpk of 1.33
or better, and a process operating between 1.0 and 1.33 is
"marginal." Many companies now suggest that their suppliers
maintain even higher levels of Cpk.
A Cpk exactly equal to 1.0 would
imply that the process variation exactly meets 3 Sigma. A Cpk exactly
equal to 1.33 would imply that the process variation
exactly meets 4 Sigma. If the process shifted slightly, and the out of
control condition was not immediately, if not sooner, detected, then
the process would produce scrap. This is the reason for the extra .33.
It allows for some small process shifts to occur that could go
undetected. The Table located here, provides an indication of the
level of improvement effort required in a process to meet these
escalating demands, where "PPM
Out of Specification" refers to the average defect level measured
in parts per million, (PPM).