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
You cannot calculate Cp if your
specifications are one sided. In other words, if you specification only
has an upper parameter specification limit or
a lower specification limit. In these cases, Cp is ignored and only Cpk
can be used.
There is some confusion about the
difference between Cp and Pk along with Cpk and Ppk. The difference is in
how you calculate the actual sigma, or standard deviation, either using an
estimate, or the actual calculations. These terms are tossed around a lot
and are sometimes used interchangeable, even though they are not. For
details see the Sigma/Standard
We have made calls out for scientific proof
that has been reviewed by peers as to the validity of the estimated way
and have as of yet to seen the data. Statistics IS a science, and as such,
all theory should be able to be validated by such peer reviews.
If a processes mean, or average, is shifted
with respect to the design or specification mean, the inherent process
capability is adjusted with a factor k, and becomes Cpk.