I came across this newsletter about X-bar and R charts. In the paragraph preceding the control chart example, the author says the following: In the bowling example, this means that you can predict what the average of your three games on any given night will be. Your average will be between 140 and 181 with a long term average of about 161. You can…[Read more]
@cseider: I agree with you, Chris. I wouldn’t feel comfortable making any kind of final judgement based on 2 or 3 data points on a control chart. I would definitely do my best to get more. However, if, for whatever reason, I couldn’t get more than 3 at that time, then, according to the authors, I could go with it and make inferences about the…[Read more]
To be clear, I was NOT suggesting that we replace the UCL and LCL with USL and LSL. I fully understand why the control limits are there, how to calculate them, and what they help tell us, the readers. They are a must on a control chart.
I was wondering what harm, if any–other than possible confusion on the reader’s part–would…[Read more]
In the book “Understanding Statistical Process Control (Second Edition),” authors Wheeler and Chambers say on page 40 “… there is no requirement regarding the number of subgroups needed before an out-of-control point can be interpreted as an indication of uncontrolled variation.” They go on to say “Thus, even if there are only two or three sub…[Read more]
Gotta ask: You guys seem to respond to most questions I’ve seen posted on this forum. Is this your day job or something? Do you guys sit around all day answering people’s questions on this forum? Every time I’ve posted a question, I know I can count on getting a response from…[Read more]
I’m confident I fully comprehend the difference between spec limits and control limits. The former is defined by the voice of the customer, whereas the latter is the voice of the process. Former is determined by the customer; latter by the calculating 3 sigma from the mean.
So, why don’t we include spec limit lines on the control chart,…[Read more]
@rbutler – Thank you for responding. I took your advice and found an article that I think will help: https://www.isixsigma.com/tools-templates/capability-indices-process-capability/process-capability-calculations-non-normal-data/ 1 year, 3 months ago
As I was playing around in Minitab 17 today, I noticed that my hand-calculated confidence interval (CI) for a dataset did not match the CI that Minitab calculated for the same dataset. (Please don’t ask why I’m hand calculating this stuff. Glutton for punishment, I guess.) In any case, my dataset has over 30 records, therefore, I was using the…[Read more]
@rbutler, I found the problem. After 10 minutes of pulling my hair out and doing the calculation over and over and over and always getting the same result of 0.61 (definition of insanity, right?), I decided to put on my Green Belt hat and do a root cause analysis. What could be the root cause of this problem? So, decided to do an experiment of…[Read more]
I’m teaching myself about testing for differences between proportions with independent samples. I use Minitab 17. I found an example in Minitab that I thought I would try to calculate by hand to see if I could get the same result to confirm my understanding of the formulas. As you can see in the example, the z-score Minitab got was 0.58. Yet, w…[Read more]
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