CONTROL CHART BASICS and the X-BAR AND R CHART +++++ EXAMPLE
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- เผยแพร่เมื่อ 26 มิ.ย. 2024
- The control chart basics, including the 2 types of variation and how we distinguish between common and special cause variation, along with how to create a rational subgroup!
We also cover the equations and constants for the X-bar and R (Range) chart, and work an example of this important control chart.
0:00 - Introduction
1:29 - The 2 Types of Variation
2:42 - How to distinguish between common and special cause variation (The Key Elements of a Control Chart)
3:44 -RATIONAL SUBGROUPING explained
5:29 - EQUATIONS for the control limits create an X-Bar and R Chart
6:53 - CONSTANTS needed to calculate the control limits for the X-Bar and R Chart
8:28 - EXAMPLE of an X-bar and R Chart
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Great video! Love seeing someone who really understands this teaching it. Too many people on youtube are just slapping +/- 3x the global standard deviation on their line charts and calling it a control chart.
Does that +/-3x global stv still senstive enough to capture the abnormal signal? what is the pro and cons for those 2 methods?
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-Andy
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@@akanksharai1366 will do! I'm sure i'll create a video on MSA here in the next 2 months or so.
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hahahaha thanks!!!! I'm glad you love it!!
-Andy
OK, this is about starting the charts but is there also a video or explanation what and how to act when the process gets out of control... when and how to recalculate limits, eliminate outliers etc...
Thanks for this intro anyway. Greetings.
perfect man
Thank you for sharing and very clear explanination on setting up the UCL & LCL. How frequent do we need to review and recalculate the Control Limit?
Hey Sunny!
That honestly depends on your process, and how stable and mature it is, and there is no hard and fast rule.
If you go-live with a control chart and your process appears to be stable over time, then that re-calculation does not have to happen that often.
If you go-live and your process is not stable, you can do both - address those sources of special cause variation, and re-calculate your control limits. But do make sure not to include any OOC (Out Of Control) data points when calculating those control limits.
In the last slide for X Bar chart I think the lower line 3.9 value should be rename as LCL not UCL. But very fine analysis.
Great catch, and you're correct, thank you though!!
thank you
You're welcome Gretel, and thanks for the comment!
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Wow thanks so much Jarod!!!!
Thanks for the video! What do you recommend when we have much larger subgroup sizes? And do you have any recommendations for how to create the centerline? I imagine it's a rolling statistic rather than something static?
Same questions too 🙋
Super helpful!!! One Question: can the Y values for the Xbar and R chart chart be negative?
Hey West, Generally the answer is no, unless you're measuring some attribute that can also be zero.
For example if you were monitoring the temperature of a cold process, you could realistically measure temperature values below zero.
However, if you're simply measuring a small attribute that can never be a negative number, and your lower control limit is a negative number, the general practice is to move that up to zero.
-Andy
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Thanks! I'm glad you like it!
Great explanation! I think it would be great if you could include some analysis for what the case would be if we include intermediate lines (+/-1 stds & +/-2 stds).
Also, what would the conclusion be if we plot a R point outside the UCL, but the x bar plot is within the UCL and LCL. My understanding would be if the R is out of control, the X bar is inaccurate. But does that necessarily mean that x bar is out of control as well, or is it simply x bar in control and R is out of control? Thanks
Hey Samuel!!! Great suggestion, that's definitely a good future video!
okay, so it is possible for special cause variation to impact your R chart, but not your X-bar chart.
That's why we have both charts, because it's important to monitor both the center of your process (X-bar) and the variation within your process.
And either chart can alert you to special cause variation!
Your the best teacher ❤❤❤
Thanks so much!
Dear Andy, I need to select a control chart to follow up our customer complaints(for CAPA triggers) , we produce single products (not batches) and there are many attributed customer complaints categories (such as: uncomfortable, broken, wrong parameters ,etc..). number of manufactured products and number of complaints varies each month. My best pick is P chart where I divide the total number of complaints received each month by the number of manufactured products from the previous month (if on January we produced 100 products and on Feb we received 10 complaints then my data point is 10/100=0.1), obviously the only control limit in my interest is the UCL. Do u think I picked the proper control chart? would like to hear your thoughts in general:)
🔥lit explanation
I was going through hard time of understanding this...
Now I thought how easy it is...
Awesome, thank you Rupesh!!!
I'm glad I can help you grow!
@@CQEAcademy Ya, it helped me in my diploma exam... 😅 Last minute studies!
@@rupeshchoudhari96 fantastic, what are you getting your degree in?
@@CQEAcademy I'm currently in last year Diploma (Mechanical)
@@rupeshchoudhari96 Best of luck!!!! I got my degree in mechanical.
Let me know if there are any other topics that you'd like to see.
-Andy
I have a few questions, I hope that you can help me.
1) How about for subgroup more than 25?
2) Can I calculate the UCL and LCL myself without referring to the constant? especially for big subgroup.
3) I have different orders for a product (mean same setting but different manufacturing date with different batches of raw material), each order with certain number of jumbo rolls, each jumbo rolls will be checked at certain meter length, each meter length will be checked several position from left to right. Should I consider all result is 1 jumbo roll as 1 subgroup and all result in 1 order as 1 subgroup.
4) If we do not use software and only excel, how should we monitor the control chart as we need to calculate the mean, UCL and LCL very frequent?
Hey Chong!
For question #1, most tables stop at 25. . I tried to google it but couldn't find anything larger. Honestly, after 25 samples, you should stop using the range value and start using the standard deviation (X-bar and S chart). That transition to the S chart usually happens around 10 samples.
For #2, no, you need to constants to calculate the UCL and LCL. You might not need the table I guess if you have the constants memorized.
For #3, the question here is all about rational subgrouping, and you want to minimize the variation within a sub-group. I doubt you could argue that all of the measurements on a single roll are 1 rational subgroup. It seems more rationale that whenever you take multiple samples at each meter in length, those measurements would be considered a single rational subgroup. Honestly without knowing your process better it's hard to say what a rational subgroup is.
#4, there are excel spreadsheets to perform all of the control chart calculations (UCL, LCL, etc), all you have to do is type in the measurement data
Hi, what is the "underlying distribution" of a process?
Is there a table with more than 100 sample size for A2 and D4? Or how can I calculate those?
Pretty cool
Thanks!
Sir, how did you find the A2 value? What is the formula behind it?
Hey Manikandan!! I love this question - they actually can be derived from the d2 values, and sample sizes.
Time stamp - 9:30
Do we not do a "X bar S chart" for this one since our subgroup size is more than 8?
Finally today I got to know about Rational Subgroup. Thanks from India.👍
Awesome, glad I could help!!!!
The video is not displaying but I can hear the sound. Do you have other source where you have this video? It would be really helpful
Is the moving range chart and x-bar chart the same?
Hello! Great explanataion!
One question:
at the rational subrouping section, how to imagine the 5 measurements / groups?
Does it mean:
5 sample parts are selected from the production same time, and 1-1 measurement is carried out on them
or
1 product is selected and that is measured 5 times, so this 5 measurements give the "subgroup"?
Thank you
Hey Great question!!! Okay, so a rational subgroup of 5 would mean 5 unique samples measured 1 time.
It's not a single sample measured 5 times. That type of variation would reflect the variation in the measurement system, not the variation in the process.
-Andy
@@CQEAcademy Oh, thank you for the quick reply!
I wondering, how this kind of UCL and LCL calculation can happen in the reality.
Imagine there is a production line, (a work station e.g. turner machine). How can I set up the control line for the process?
Should the control lines be there, before he begins the job?
As planned, every half hour a sampling will be measured (e.g 3pcs/sampling). After 4 hours there will be 8 subgroup- measurement results. After I have 8 result, i calculate the control line.
1. How the process will be monitored before the control lines' calculation, in the first sevaral hours?
2. What is the meaning of the control lines' calculation, if the next and next and next subgroups' result will again and again change the UCL and LCL values?
Sorry for the detailed question, i just would like to understand this very well.
@@surnamegm Hey There,
Okay, so generally, you want to collect around 25 sub-groups to calculate the control limits. Until you have control limits in place, just consider the chart to be a "run-chart" with no control limits.
Also, before calculating the control limits using those first 25 sub-groups, you want to feel reasonably certain that the process was only experiencing normal (inherent) variation.
You should not re-calculate the control limits for each new data point.
However it's not unreasonable to re-calculate the control limits if a change is made to the process.
This is something that you'll get better as assessing over time (the need to re-calculate the control limits). It's hard to create rules for this scenario because every process is different.
Hopefully that helps!
-Andy
@@CQEAcademy Thank you very much!
@@surnamegm You're welcome!
Thanks for your videos, it really helps a lot. A quick question though, what if I measure each sample 2 times? Do I need to count each of them as 2 samples or as 1 sample but using their average measurement?
Hey Agra, I assume that when you take those 2 measurements, that you're measuring the same characteristic - not two different things (dimensions, etc).
I haven't run into that situation before - why would you measure the same sample twice?
@@CQEAcademy Yes, it is the same characteristics but 2 replications. I am not really sure why there are 2 replications because I just retrieved it from old data, probably because there is a doubt in the gage precision
@@agradhanurwedhasakti4865 That's what came to mind when I first read the two replicate measurements, that someone was worried about gage precision.
Honestly, what I'd do is run a Gauge R&R, and if the R&R is acceptable - go down to only a single measurement!
That might be a huge form of waste (over-processes) by measuring the same sample twice.
And if the gauge R&R is bad. . . it might be time to find a new gauge - too much precision should not be overcome by repeated measurements.
Thanks for your video
Is there video longer than this
You're welcome Peino! I do cover this topic in alot more detail in my course, The CQE Master Class
Thanks
Excellent❤❤❤,
One question
What if we have different tolerance for each subgroup,
For example for subgroup 1 we have +/-2 and for subgroup 2, +/-3 and so on,
Have we calculate process capability in this condition?❤❤❤❤❤❤
I wish these things were taught in business degree classes, that would save billions and many companies from closure.
Worked one time at a company, where they have appointed a "manager" to be a process manager, who did not know anything about spc nor the variation nor the chart technique, that person did not even know how to draw a flow chart.
Needless to say, his whole "procedure of management" went then into the direction of fire fighting and blaming of the workers including constant firings, etc.
Hire and fire mentality in sales departments is another example of lack of knowledge and understanding in that field.
teaching level 😊
Thanks!!!!!!
is the time period between each sub groups a standard. For example if the time period between each sub groups are 24 hours then what is the use of using control charts because if the time period is too long we cannot estimate fast how our process is under control. Please answer sir
Hi! May I ask, where did you make your x-bar and r chart? Is there any particular application or website you recommend for me to use in making a control chart? Thank you!
Hey There!! I made that control chart in Excel
@@CQEAcademy Oh I see, thank you!
@@Name-ur6oj You're welcome