Cpk là gì

Cp, Pp, Chành động, and Pvõ thuật are capability và performance indicators for your process. Is a process capable? Is the process acceptable? How is the process actually behaving vs how it theoretically could be behaving? These are questions you can ask during a DMAIC project during both the measure phase (if you have sầu good, existing data) or in the control phase after you have implemented your changes.quý khách hàng đã xem: CPK là gì?

This article deals with an overall examination of Process Capability và Performance (Pp, Pđánh nhau, Cp, Cpk).

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If you’d like more depth including calculations, etc, see these articles:

To begin to answer these questions it all starts with customer requirements and the follow up with what we know mathematically about the variation of the process.

Cp, Chành động, Pp và Phành động are all parameters (indices) that can help us to lớn understvà how our process is operating relative sầu khổng lồ the specifications, or in other words, they measure how cthất bại our process is running lớn its specification limits.

For requirements we measure the process specifications. To determine the answer khổng lồ those questions we see how wide the process dispersion (spread of values) và how centered the process is relative to those specifications.


Process spread vs centering

On the mathematical side we require the process to follow a normal distribution so we can take advantage of the properties there. Before using these indices khổng lồ see how capable và acceptable your process is, chạy thử your process for normality. If it’s not normal, see if you can apply a transform lớn get it there. If not, you may have lớn use non-normal distributions and methods (not part of the Six Sigma Blachồng Belt curriculum. – See the Master Blaông xã Belt guide.)

Cp, Ckungfu, Pp, Pkungfu Practice Questions & Z Charts


Practice makes perfect! Free Cp, Cchiến tranh, Pp, Pkungfu practice questions.

When to lớn Use Pp, Pđánh nhau, Cp, and Cpk

The main difference between Pp and Cp on one side và Pđánh nhau and Cchiến đấu on the other side is whether we use complete mix of data for calculation (Pp và Ppk) where we calculate real performance of the system, or we use sample (pre-production, batch, logical subgroups) where we calculate capability of the process. In equation for Pp and Phành động we use standard deviation based on studied data (whole population), & in equation for calculation Cp & Cchiến tranh we use sample deviation or deviation mean within rational subgroups


When to lớn use Pp Pvõ thuật Cp Cpk

Differences Between Cp và Pp

The biggest difference between Cp và Pp is how the Standard Deviation is determined. Pp we use sampling & have sầu khổng lồ calculate an estimated standard deviation of the sample. In Cp we are assuming a stable process & will likely have enough data lớn calculate a true standard deviation.

What Can Process Capability và Process Performance Tell Us?

Process Capability Analysis tells us how well a process meets a set of specification limits, based on a sample of data taken from a process. It can be used to lớn establish baseline for the process and measure the future state performance of the process for comparison.

Here we want to use Cp for new processes or Pp for processes that are understatistical control. A good analogy is parking a car in a garage. If the car is bigger than the garage, it will not fit within the specification limits và thus is not capable – this case is clear. What about a case when our car is skinnier than the garage you’re trying khổng lồ park it in (the specification width)? Can we consider our parking process as a capable one? It depends on our specification & the width of the car. Are we satisfied with the probability of 99% that we will park our oto, without hitting the wall? This mean that one in a hundred attempts lớn park our car will hit the wall of the garage!

One way to lớn calculate Process Capability is through number of defects per opportunity. This is usually used for discrete data & in manufacturing is usually acceptable number of 3.4 Defects Per Million Opportunities (DPMO)


The other way to calculate Process Capability is through use of statistical methodology. Processes with normal distribution of data, have sầu restrictions from one or both side of the curve, based on the Specification Limits (USL- Upper Specification Limit và LSL- Lower Specification Limit):


Shaded areas represent the probability of defects & our goal is to minimize the kích cỡ of the shaded areas.

Is a Process Capable?

Process capability analysis is the determining if a process can meet specification.

Here we want khổng lồ use Pp for new processes or Cp for processes that are under statistical control. A good analogy is parking a oto in a garage. A process that is capable indicates the car (process) is skinnier than the garage you’re trying lớn park it in (the specification width). If the oto is simply much bigger than the garage, it is not enough khổng lồ be capable of fitting within the specification limits và thus is not capable.

See how khổng lồ calculate Cp.

See how to lớn calculate Pp.

Cp, Ckungfu, Pp, Pchiến đấu Practice Questions và Z Charts

Practice makes perfect! Free Cp, Ckungfu, Pp, Pđại chiến practice questions.

Is the Process Acceptable?

A capable process does not necessarily mean it is acceptable. Using the garage example, the car may have crashed into the side of the garage, or may have sầu missed it completely. This point reflects how centered a process. We can measure this using Pchiến đấu or Cchiến đấu.

See how to calculate Pchiến đấu.

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A lưu ý on a Process Being Both Capable and Acceptable

A process is said to lớn be capable & acceptable when it meets customer requirements. Having zero defects is usually unrealistic & not usually a part of customer requirements. A process could be capable, but not acceptable because it is poorly centered và ends up outside the spec limits (ex car could fit in the garage, but the process for parking it is could have it just as easily kết thúc up in your front door as where it should be).

Six Sigma Process Capability & Performance Limits

According khổng lồ Six Sigma philosophy, Cp or Pp & Ckungfu or Pđánh nhau should be greater than 1.50. From a technical standpoint, Six Sigma deems a process being acceptable only after achieving a maximum defect rate of 3.4 parts per million opportunities. There is a direct correlation between Cđánh nhau or Pvõ thuật and parts per million defects because only Cps or Pps và Cpks or Ppks that are greater than 1.50 are achieving this goal (3.4 Defects per million).

Control Limits, Specification Limits & Process Capability

Keeping a process in control limits is not sufficient lớn achieve sầu good quality. Quality is always dictated by the user. For that we use specification limits when assessing process capability. The process specifications used in process capability are the voice of the customer và control limits of Statistical Process Control are the voice of the process.

It is very possible to have sầu great process control (indicated on a control chart) yet have poor process capability. That would be a mismatch of where the process is centered versus where the customer wants it lớn be centered. Either that or the process has more variability than the customer desires.

Pp vs Cp (Capability Indices)

Both Cp và Pp are a monitoring indices for the spread of your process compared lớn the specification spread.

Cp is used when a process is under statistical control. Pp is used when a process is initially starting out.

Uses for Cp & Pp

Used to lớn see if a process is capable.Neither are concerned with centering. Cp or Pp takes inlớn account dispersion, NOT centering. Since Cp does not consider process centering so it cannot be used lớn determine if a process is acceptable.Cchiến đấu và Pchiến tranh both take centering inlớn account, so they can be used to see if a process is acceptableCp & Pp are NOT concerned with the control of a process. The term “control” infers an element of time and Cp or Pp is merely a snapshot of the process variation at a given point in time.For example, a Cp of Can never be 0.If Cp or Pp >1, Tolerance is > spread, Process has potential lớn be capable (depending on centering).A CP. of >1 doesn’t mean quality One could have sầu a Cp of 400 & be producing 100% defects, because the process was not centered within the specification limits.If Cp or Pp tolerance, & the variation will never fit & the process will never be capable.A Cp of If 0Ex. Cp 0.70 = 70%. A Cp of 0.70 means the specifications are 70% of the process.

Pchiến đấu vs Cchiến tranh (Acceptable Performance)

Cpk and Phành động are ways to measure the capability of a process because they are monitoring indices for process centering.

Uses for Cp và Pp: Is The Process Acceptable?

Cchiến tranh or Pđánh nhau takes into account centering & thus can be use lớn determine if a process is acceptable.

Ways khổng lồ measure the capability of a process.Reflect the actual performance of the process.Both Cchiến đấu và Phành động can be a negative numbers.It means that, on average, the process is not meeting customer requirements.

Can Cđại chiến or Phành động be Negative? What does a negative sầu Cvõ thuật or Pđánh nhau Indicate?

Yes, it is possible for Cđánh nhau & Pkungfu to lớn be negative sầu. Let’s look at the equation – for simpliđô thị we will use the Pkungfu.

Phành động = / 3 s

It is impossible lớn have a standard deviation be negative sầu so that would mean that x(bar) was larger that the Specification Limit. In other words, the process average is out of specification.

This could be an indication that the process mean has drifted over either the upper specification or the lower one. This is not good because it means that the process is not meeting customer requirements.

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Pp, Phành động vs Cp, CPK

Interesting video on assessing Pkungfu và Cđánh nhau in Minitab.

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