Process Capability is the name given to Cpk. This is typically used when a process is in statistical control. With a mature process that has been around for a long, this frequently occurs. Using either the Moving Range, Range, or Sigma control charts, the process sigma value is calculated.
Theoretically, Cpk will always exceed or be equal to Ppk. When the sample size is small and the data only covers a brief period of time, anomalies might occur where the standard deviation is overestimated by the R estimator, causing Cpk to be less than Ppk. Since the long term uses all the data, not just two bits from each subgroup, there can never be less variation over the long run.
When using the Pp & Ppk technique, evaluating process capability using Cp & Cpk mirrors what is done (and why it is done) when using that approach. The primary distinction is that Cp & Cpk are used only when a process has attained statistical control or stability.
The Garage Parking Comparative Example
Imagine the boundaries of the client specification as the garage walls, where you must fit your car. Overstepping certain boundaries will result in a crash, which will make the client unhappy.
When there is a lot of variety in your process, the process average is dispersed. Not good for any process, not ideal for parking a car. Working on decreasing variation and centering will increase the likelihood that your parking operation will be successful.
Nothing you do to center the procedure will help if the car is too wide for the garage. You must alter the procedure’ dispersion (make the car smaller.)
It doesn’t matter if you park the car exactly in the centre if it is much smaller than the garage; it will fit and you have space on either side. One of the reasons the six sigma philosophy emphasizes eliminating variation in a process is due to this.
You should be able to park the automobile quickly inside the garage and fulfill the needs of the customer if your process is under control and rarely varies. Cpk explains the correlation between the dimensions of the car, the size of the garage, and the distance from the center of the garage that the car was parked.
How to Determine Cpk
The standard deviation of the specification limits from the process’s center is indicated by the Cpk measurement. You can perform this graphically for some procedures. Some call for an equation.
You must compute a Z score for the upper specification limit (referred to as Z USL) and a Z score for the lower specification limit in order to determine Cpk (called Z LSL).
It should come as no surprise that the value of those limitations, the process mean, and the standard deviation are all factors in the Z calculation given that we are seeking to determine how many standard deviations fit between the center line and the specification limit.
Cp is a contraction. In reality, there are two parts: the upper and lower portions, referred to as Cpu and Cpl, respectively. These are their equations:
- Cpl = (Process Mean – LSL)/(3*Standard Deviation)
- Cpu = (USL – Process Mean)/(3*Standard Deviation)
- Cpk is merely the smallest value of the Cpl or Cpu denoted: Cpk= Min (Cpl, Cpu)
How to Calculate CPK formula With Excel
If you’re going to calculate statistical data, learning how to calculate CPK in Microsoft Office Excel can save you time. CPK is used to gauge how evenly distributed the sample data is with respect to a given limit. You must use the “Average” function to determine the average of your sample data in order to calculate CPK. Additionally, you must use the “STDEV” function to determine the standard deviation. This function calculates how far the values deviate from the mean.
- Launch Microsoft Excel and type “Data” in A1, “Upper Limit” in B1, “Average” in C1, “StDev” in D1, and “Cpk” in E1.
- Type “1” in A2, “2” in A3, “3” in A4, “4” in A5, “5” in A6, “6” in A7, “7” in A8, “8” in A9, “9” in A10, and “10” in A11. Type “15” in B2.
- Include the following formula in C2 to determine the data’s average. =AVERAGE(A2:A11)
- Include the following formula in D2 to determine the data’s standard deviation. =STDEV(A2:A11)
- Compute CPK by using the following formula in E2 and using the values for the upper limit, average, and standard deviation. =((B2-C2)/(3*D2))