Performance Indices Pp
and Ppk are often referred to
as 'long term' process capability measures.
This is because the standard
deviation used in their calculation
is calculated using the 'Root Mean Square
method which is affected (inflated) by shifts
in the process mean.
The Process Capability indices Cp
are referred to as 'short term' process
capability measures because the standard
deviation used in their calculation is calculated
using the Range Method which is not affected
by changes to the process mean.
There is much confusion and debate about
the meaning of short and long term capability,
so use these terms with care and be sure
that everybody has the same understanding.