![]() ![]() ![]() #CPK CALCULATIONS IN EXCEL FREE SOFTWARE SOFTWARE#The Cpk analysis above was made using SPC for Excel, a simple but powerful software for statistical analysis in the Excel environment.Kutools for Excel Solves Most of Your Problems, and Increases Your Productivity by Our SPC Knowledge base has multiple publications on process capability if you would like more details. That metric is Ppk, which will be the topic of our next blog. Ppk? Yes, there is another commonly used process capability analysis metric. In this example, only the Cpk and Ppk process capability indices are included. The SPC for Excel software allows you to choose which statistics to include. ![]() This means that essentially all the process output is within spec.Īs shown in the chart above, there are usually some statistics included with the Cpk analysis. In this example, you can easily see that the process output is within the specification limits. What does a Cpk analysis look like? An example of a capability analysis from the SPC for Excel software is shown below.Ī Cpk analysis shows the histogram of the process output with a normal curve and the specification limits added to the chart. If these two things are true, then you can do a process capability (Cpk) analysis. The only way to find this out is to create a control chart based on the process output. Second, the process must be in statistical control it must be consistent and predictable. The calculations of Cpk are based on that assumption. First, the process output must be reasonably normally distributed. There are two things that must be true before you can calculate a Cpk value. If the LSL less than 3 σ below the average, then there is material out of spec on the low side and Cpl < 1.Ĭpk is simply the minimum of Cpu and Cpl. This, in turn, means that there is essentially no out of spec material on the high side. If Cpu > 1, this means that the USL is over 3 σ from the average. 3 σ is a range from the average to the average + 3 σ. USL-the average is measure of how far the USL is from the overall average. Essentially all the process output lies in a range of ± 3 σ of the average. The bell-shaped (normal) curve represents the output from the process. The chart below shows the concept behind Cpu and Cpl. Note that σ is not the calculated standard deviation you get from using the basic formula (like the formula STDEV does in Excel). Where Cpu is the capability based on the upper specification limit (USL), Cpl is the capability based on the lower specification limit (LSL), X is the overall average, and σ is the estimated standard deviation from a range control chart. Numerically, Cpk is expressed as the following: This gave rise to numerical expressions to compare the range of process output to the specification range. Since you are comparing two ranges – the range of output from the process to the range of the specs - it is natural to ask questions about how much these two ranges are different or overlap. ![]() Process capability is the range of values that you can expect to get from the process over an extended time period.Ī process is capable if the range of expected values fall within the specification limits. #CPK CALCULATIONS IN EXCEL FREE SOFTWARE SERIES#Our first blog in this process capability series explored the simple question “what is process capability?”. ![]()
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