Cp And Cpk Versus Pp And Ppk

 

Universidad Panamericana  
 

 

“Cp and Cpk vs. Pp and Ppk in the manufacturing industry.” 
Juan Lerdo de Tejada 
Comparing these indices after describing all of their information, to conclude which one is more 
reliable and has a bbeter use in the manufacturing industry. 

Statistics                                                                    18/May/12 

“Cp and Cpkversus Pp and Ppk”
In this essay I will discuss, relation, compare, describe the process capability indices
(Cp and Cpk) and the Process performance indices (Pp and Ppk) and with my own
conclusions decide which of different indices has better use in the industry of
manufacture, mainly comparing Cp vs. Cpk and Pp vs. Ppk, concluding C vs. P.
In the global world we now a day live there is anextreme competition among the
manufacturing industries, quality and control are basic for an industry; it is a fact no
need arguing why, through this essay these indices will show which one of them has a
better and more efficient use in the manufacturing industries. Variance in the process
and in the product is our principal enemy.
It is important to define the terms and explain the basicconcepts that will be used
during this essay:
• Process capability: basically “indicates how much natural variation a process
undergoes in relation to the specification limitsi”
• Performance rate (Pp): Measures long term ability of a process to meet specs.
• Cp: Value of the theoretical capability of the process.
• Cpk: Value of the current process performance able to diagnosedecentralization.
• Ppk: Value of the long-term ability of a process to meet specs able to diagnose
decentralization.
• Normal Distribution: Distribution that normally distributed.
• Specification limits: If the product is outside these limits is not to be delivered.
• Control limits: Limits used for triggering corrective actions.
• UCL: Upper control limit.
• LCL: Lower control limit.
• USL: Upperspecification limit.
• LSL: Lower specification limit.
• Rational Subgroups: simple in which the differences in the data within a
subgroup are minimizes and the differences between groups are maximized.
• C`s indices: Include Cp and Cpk.
• P`s indices: Include Pp and Ppk.

It is very important to explain before we start the relation of the indices, what are the
specification limits, theselimits are always negotiated with the customers which are
defined to decide whether a product is to be delivered or not, these limits can be
defined multiple times for different customers, markets or functions, because not all
product purposes need the same quality. Another tool helpful as we saw in class is the
Control limits; these limits are used for triggering corrective actions. If theprocess
runs out of the control limits, a production system should signal this event
inmeditially and someone responsible has to start corrective actions to get the process
back to target or the deviation back to a normal level. The control limits need to be
defined left and right (or above and below) the target to make sense for corrective
actions. For the purpose to start corrective actions,the control limits also have to be
defined between the specification limits. If the control limit triggers an event outside
specification, it’s too late for any meaningful actions. Several control limits can be
defined if necessary to separated different levels of escalation or different levels of
immersive corrective actions.
Before we dig deeper it is important to fully understand what aprocess of
manufacturing is: “Industries which involve in the manufacturing and processing of
items and indulge in either creation of new commodities or in value addition. The
manufacturing industry accounts for a significant share of the industrial sector in
developed countries. The final products can either serve as a finished good for sale to
customers or as intermediate goods used in the...