1. Process capability CP
process capability compares the output of an
in-control process to the specification limits by
using capability indices. The comparison is
made by forming the ratio of the spread
between the process specifications (the
specification "width") to the spread of the
process values, as measured by 6 process
standard deviation units (the process "width").
2. We are often required to compare the
output of a stable process with the
process specifications and make a
statement about how well the process
meets specification. To do this we
compare the natural variability of a stable
process with the process specification
limits.
Process Capability Indices
3. A process where almost all the
measurements fall inside the specification
limits is a capable process.
4. What is cpk
nterpreting Cp, Cpk
“Cpk is an index (a simple number) which
measures how close a process is running
to its specification limits, relative to the
natural variability of the process. The
larger the index, the less likely it is that
any item will be outside the specs.” Neil
Polhemus
5. cpk measures how close you are to your
target and how consistent you are to
around your average performance
6. What is pp
n process improvement efforts, the
process performance index is an estimate
of the process capability of a process
during its initial set-up, before it has been
brought into a state of statistical
control.[1]
7. Formally, if the upper and lower
specifications of the process are USL and
LSL, the estimated mean of the process is
, and the estimated variability of the
process (expressed as a standard
deviation) is , then the process
performance index is defined as:
8. is estimated using the sample standard
deviation. Ppk may be negative if the
process mean falls outside the
specification limits (because the process is
producing a large proportion of defective
output). Some specifications may only be
one sided (for example, strength). For
specifications that only have a lower limit,
; for those that only have an upper limit, .
9. What is ppk
PPK is used for long term. When the
Process Capability Index shows how a
process will do in the future, the Process
Performance Index shows how a process
performed in the past. It can also be seen
that the process will be in a state of
control in CPK where as it is not like that
in PPK.
10. PPK or Process Performance Index helps in
verifying if the sample generated is
capable to meet Customer CTQs.
When PPK stands for Process Performance
Index, CPK stands for Process Capability
Index.
11. Calculating the sigma quality level of
the process
essential first step in "6 sigma" is to define clearly
what the customer wants, needs explicit. We
call these requirements "critical features of quality."
The next step is to determine the probability of
defects or failuresthat occur in relation to these
characteristics.
One way is through classical statistical methods, such
as a 6 sigmaprocess will have a Cp = 2. That is, when
it is centered, if one takesthe distance between the
sample mean and the specification limits, this
will 6 standard deviations (usually identified with the
Greek lettersigma).
