18 different charts? 18 sample points over 28 days

I've got myself very confused.
If I have 18 different sample points throughout a process and sample each of them once a day for 28 days how to I chart stability of the process. Do I need to do 18 charts each with a sample size of 1?
Thank you so much for any help.
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Jen Kirley

Quality and Auditing Expert
Welcome to The Cove ricen!

What process is this that has 18 different sample points? What are these sample points? Which ones are critical and which are not?

I am also wondering about your sample size of 1 per day. How big are your lots or batches? I am wondering how a single sample would allow you evaluate stability in your process. That said, I can understand it if you make chemicals in large batches, one per day.
Thank you so much for taking the time to reply.
This is a purified water distribution loop.
18 sample points along the loop. All equally critical. I have 28 days of data with tests results everyday at each point for conductivity, total organic compounds, total microbial count and residue on evaporation.
How best to present all this data and show the system is in control? Previously my company has taken the daily mean of all sample points and plotted an x-bar & s charts using the Stdev of the daily mean of all points. One graph for 28days of conductivity,bone for toc etc. We superimpose our acceptance criteria onto the x-bar graphs, which I'm pretty sure is a no no.
I'm just not sure where to start with this one.
Thank you for the warm welcome.
I should add that this is an academic exercise. I am the system owner and the system is fully validated. I just want to learn how to apply lean six sigma principles to the data before I suggest their use to my employer. I'm open to any suggestions. I am a green belt but am new to the company and there are no Black belts here to ask for guidance. We are in the pharmaceutical space.

Thinking again to the criticality of all 18 sample points.: I'm open to suggestion regarding risk analysis to reduce the test points. We do 18 points because "that's what we've always done".

Extra info: The water is constantly circulating around the loop. The samples are taken manually by one person walking from point to point over approx 2 hr period.

Bev D

Heretical Statistician
Super Moderator
First I strongly suggest that you read the articles of Donald Wheeler. These are free and can be found at quality digest. The most ritual ones are also available for free at his website spcpress.com. You must understand the concept of homogeneity and rational sampling to properly select a control chart design. I seriously doubt that you were taught this as a green belt - even black belts are rarely taught this.

Without looking at the data we cannot really advise on the best way to chart it. It all depends on the homogeneity of the 18 points. A third research article would be the 3 Way Control Chart as explained by Donald Wheeler. If you can post the data and the charts we can provide more help…
Thank you Bev, I will check those out. (by the way I LOVE your signature!)

How can I post the data? I don't see how to attach a file.

Bev D

Heretical Statistician
Super Moderator
Here are direct links to the best articles that might help you:

Prevette, Steven, “Liars Figure and Figures Lie”, ASQ Newsletter October 1999. Copy provided via Elsmar Cove Quality Forum by Steve Prevette.
@Steve Prevette perhaps you could provide a link to your article? or repost it?

Wheeler, Donald, “Why We Keep Having Hundred Year Floods”, Quality Digest, June 2013, Why We Keep Having 100-Year Floods

Wheeler, Donald, “The Secret Foundation of Statistical Analysis”, Quality Digest, December 2015 The Secret Foundation of Statistical Inference

Wheeler, Donald, “Statistics 101 and Data Analysis”, Quality Digest, March 2016 Statistics 101 and Data Analysis: an Example

Wheeler, Donald, J., “Foundations of Shewhart’s Charts”, SPC Tool Kit column, Quality Digest, October, 1996 http://www.qualitydigest.com/oct96/spctool.html

Wheeler, Donald J., “The Empirical Rule”, Quality Digest, March 2018 The Empirical Rule

Wheeler, Donald, “The Right and Wrong Ways of Computing Limits”, Quality Digest, January 2010 The Right and Wrong Ways of Computing Limits

Wheeler, Donald, “Good Limits from Bad Data I”, Quality Digest, March 1997 SPCTool

Wheeler, Donald, “Good Limits from Bad Data II”, Quality Digest, April 1997 SPCTool

Wheeler, Donald, “Good Limits from Bad Data Part III”, Quality Digest, May 1997 SPCTool

Wheeler, Donald J., “Myths About Shewhart’s Control Charts”, SPC Tool Kit column, Quality Digest, September, 1996 http://www.qualitydigest.com/sep96/spctool.html

Wheeler, Donald J., “Myths about Process Behavior Charts”, Quality Digest September, 2011 Myths About Process Behavior Charts

Wheeler, Donald, “What is a Rational Subgroup?”, Quality Digest, October 1997 SPCTool

Wheeler, Donald, “Rational Subgrouping”, Quality Digest, June 2015 http://www.qualitydigest.com/inside/quality-insidercolumn/060115-rational-subgrouping.html

Wheeler, Donald, “Rational Sampling”, Quality Digest, July 2015 Rational Sampling

Wheeler, Donald, “The Chart for Individual Values”, https://www.iienet2.org/uploadedfil...ransition/The Chart For Individual Values.pdf

Wheeler, Donald J., “A History of the Chart for Individual Values, The ultimate in homogenous subgroup”, Quality Digest July 2017 https://www.spcpress.com/pdf/DJW317.Jul.17.History of XmR Chart.pdf

Wheeler, Donald, “The Three-Way Chart”, Quality Digest, March 2017
Fantastic thank you. That's my Christmas reading sorted.
I have the raw data in an excel sheet. I can't see how to attach it.

Steve Prevette

Deming Disciple
Super Moderator
I believe there is a limit on the number of posts you need to make before you can post an attachment. You could "private msgs" to Bev (and myself if you want) in the meanwhile. The newsletter archive for my ASQ section is Columbia Basin ASQ (asq614.org) but it only goes back to 2006, so I don't think the article is online. I'll need to search my files, I may still have a copy of the newsletter since I was also the editor of the newsletter at the time the articla was likely posted.
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