@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]
tgause replied to the topic Why Shouldn’t We Include Spec Limits in Control Charts? in the forum Tools & Templates 1 week, 2 days ago
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]
tgause replied to the topic Why Shouldn’t We Include Spec Limits in Control Charts? in the forum Tools & Templates 2 weeks, 3 days ago
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]
tgause started the topic Why Shouldn’t We Include Spec Limits in Control Charts? in the forum Tools & Templates 2 weeks, 4 days ago
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]
tgause replied to the topic How to Determine Process Capability with Non-Normal Data? in the forum Tools & Templates 5 months, 2 weeks ago
@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/
Tom Gause replied to the topic Attn Minitab 17 Experts: Calculating Confidence Intervals in Graphical Summaries in the forum Tools & Templates 5 months, 4 weeks ago
@Minitab – First, thank you for taking the time to respond to my question and for clearly explaining what was going on. Second, based on your response, I realized I had made an error in my post. I should have said the normality test “failed to reject” the null hypothesis.
Tom Gause started the topic Attn Minitab 17 Experts: Calculating Confidence Intervals in Graphical Summaries in the forum Tools & Templates 6 months, 1 week 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]
Tom Gause replied to the topic Help with Testing for Differences Between Proportions with Independent Samples in the forum Tools & Templates 6 months, 3 weeks ago
@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]
Tom Gause started the topic Help with Testing for Differences Between Proportions with Independent Samples in the forum Tools & Templates 6 months, 3 weeks ago
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]
Allow me to clarify the situation for you, so you’ll better understand what I’m asking and why I’m asking it. My apologies for not being clear previously. To quote Julie Andrews in the Sound of Music, “Let’s start at the very beginning, a very good place to start.”
I’m coaching a colleague on a project she’s leading. She approached…[Read more]
Thank you very much for your detailed explanations. I greatly appreciate the time and effort you made to articulate your responses. If I may, I’d like to continue this discussion and request your further reply.
You stated previously, “The fact remains the minimum number of samples needed to make inferences about a population is 2.…[Read more]
I’m hoping someone can clarify this for me. Maybe I’m not understanding something here, but I keep reading that a “large sample size” is one that is greater than or equal to 30. And with a sample size of n>=30, you are able to make inferences about the population from the sample. If this is correct and you only need a minimum of 30, then why do we…[Read more]
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