I received a question after my last blog post asking me to clarify the concept of within and between subgroup variation which is used in calculating Cpk, Cp, Cr, Ppk, Pp, Pr and other statistics. Here is an example I used to help explain the differences.
Let’s say that every day I run about 30 minutes with my chocolate labrador, Cadbury (pictured below).
While running, I decide to measure how fast we are going. I measure the speed (pace) three times throughout the run: toward the beginning, the middle, and the end of the run. This data tells me a few things:
1. The pace at the beginning, middle, and end of the run.
2. The average pace we keep. This average pace is also called an X-bar.
3. The difference between the fastest pace and the slowest pace, also called the range.
4. Cadbury, like me, has a lot more energy at the beginning of our run than at the end.
I often get asked SPC application questions such as: Can Ppk be larger than Cpk? Can Cpk be larger than Cp? Do you recommend CPM, Cpk, Ppk or something else? I prefer to answer these process capability questions with simple one-word answers, but that doesn’t usually satisfy the quality zealot’s curiosity. So here’s the long and the short of one of the questions.
In short, Yes, Ppk can be larger than Cpk. If you are doubtful, or consider yourself a quality zealot, grab a cup of joe, and read on.
I recently received an e-mail from SPC Press about a new book Donald Wheeler wrote, Twenty Things You Need to Know. Don has written many books on SPC, measurement systems analysis, Six Sigma, and other quality topics.
Although I have not yet read Twenty Things You Need to Know, I can tell you that his other books (such as Understanding Statistical Process Control and Understanding Variation) are worth reading. Okay, they don’t exactly read like a good novel that you can’t put down, but they serve as outstanding quality statistics reference sources.
If you are interested in his new book, you might want to look at an excerpt which is available through this link: http://www.spcpress.com/pdf/three_questions.pdf
In this excerpt, Wheeler states
“…these three questions define the essence of how to get things done.”
Recently I was given an Excel file that contained some data for the time it took to complete a task. The data had a column that listed the “time” and the data was in time-of-day format. I wanted to make a control chart showing how long, in minutes & seconds, a task took. Unfortunately, Excel categorized this as a Date/Time field, which can not be easily charted. Since the column listed the time the task took in hours:minutes:seconds, I needed a way to convert this to a number that I could chart on a control chart.