Cpk and Cp are short-term capability measures for a particular process. Six Sigma describes process quality in terms of sigma. This allows us to easily talk about the capabilities of different processes using a common mathematical framework, it’s also easy to use Cpk in excel, learning and applying it as well. It allows us to compare orange and apple processes. The distance between the mean and the nearest specification limit is divided by (3 x standard deviation).
Process Capability
Although this is a lengthy article, I felt it important to keep Cpk and Cpk separate. Cpk first, and then Cpk. Crib notes are available on the meaning of equations in real performance terms, how you should be able tell about a process depending upon Cp and Cpk value and other details. Please let me know if you don’t find what you are looking.
Cpk and Cpk are known as Process Capability. Ppk and Ppk are known as Process Performance. In both cases, we want to see if the process meets Customer CQs (requirements).
For Process Capability, Cpk and Cpk can be used in excel. This is used when a process is subject to statistical control. This is often the case for mature processes that have been around for some time. The process capability is determined using the moving range, range or Sigma control charts.
For Process Performance, PPk and Pp are used. This is used when the process is too new for statistical control. Ex. Ex. Process Performance uses sample sigma for its calculations.
Cpk should always be greater or equal to Ppk in theory. Anomalies can be seen when the sample size and data span a limited time. R estimates will overstate standard deviation, making Cpk in excel smaller than Ppk. This is false. There can never be less variation over the long-term, as long as all the data is used and not just two subgroups.
The Ppk & Ppk approach allows you to evaluate process capability using Cpk and Cpk. Cp & Cpk are only used after a process has achieved stability or statistical controls.
What is Cpk?
Analogy of Parking a Car in a Garage
The walls in your garage are where your car must fit. They become customer specifications limits. You will crash if you exceed these limits and your customer won’t be happy.
If your process is highly variable, it means that the average value of the process is not consistent. This is not good for parking cars, nor for any other processes. You can make your parking process more efficient by reducing variation and ensuring that it is centered.
The process capability index (Cpk), a statistical tool that measures the ability of a process, to produce output within specified limits for customers, is called a statistical tool. It measures the producer’s ability to produce a product within the customer’s tolerance range. Cpk can be used to determine how close you are to a target and how consistent your performance is relative to that target. Cpk will give you the best-case scenario for your current process. Cpk in excel can also predict the future performance of the process, provided that performance remains consistent over time.
You can’t center the car if it is too large for the garage. The dispersion of the car must be changed (make it smaller).
It doesn’t matter how small your car is, as long as you place it in the middle of the garage, it will fit. There will be plenty of space on each side. This is why the six-sigma philosophy emphasizes eliminating variation in a process.
You should be able to park your car in the garage easily if you have a system that works well and is easy to follow. C shows you the relationship between car size, garage size, and distance from the garage center.
How to Calculate cpk in Excel
Learning how to calculate CPK in Excel can save you time if you’re planning on doing statistical data calculations. CPK is used to measure how centered the sample data is from a specified limit. In order for you to calculate CPK you need to calculate the average of your sample data by using the “Average” function. In addition, you need to calculate the standard deviation using the “STDEV” function. This function measures how far the values are dispersed from the average value.
cpk in excel: Step 1
Launch Microsoft Excel and type “Data” in A1, “Upper Limit” in B1, “Average” in C1, “StDev” in D1, and “Cpk” in E1.
cpk in excel: Step 2
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.
cpk in excel: Step 3
Add the following formula in C2 to calculate the average for the data:
=AVERAGE(A2:A11)
cpk in excel: Step 4
Add the following formula in D2 to calculate the standard deviation for the data:
=STDEV(A2:A11)
cpk in excel: Step 5
Calculate CPK using the upper limit, average, and standard deviation values by adding the following formula in E2:
=((B2-C2)/(3*D2))
