RSloop

Ganesh,
 
1)      No, one purpose of a control chart is to detect if a process is in control.  You don’t have to wait for a “stable” process to begin charting or even to calculate control limits.
2)      Sampling frequency basically depends on how fast the process changes.  There are mathematical methods, but it all comes down to sampling oft…[Read more]

Jay,
There is an alternative to the standard gage R&R methods that is much more realistic.  Don Wheeler has been teaching it for years and it is now included in the AIAG MSA handbook as an approved method.  If you have access to the handbook it is listed as the Average and Range method starting on page 99 of the 2002 edition.  Dr. Wheeler also ha…[Read more]

Ron,
 
The formula that I posted is a standard method of estimating the sensitivity of a X bar R chart at different sample quantities.  It’s used in most control chart training courses and in the majority of statistical handbooks.  I really didn’t think that there was anything controversial about it when I made the first post. After some of th…[Read more]

Good answer Gabriel and a good point about the probability of a good sample or bad sample being the same.
Rick

NightCrawler,
The Hypergeometric Probability Distribution is used when samples are taken (without replacement) from a finite population.  
I can’t paste the formula here, but if you’d leave an email I’ll be glad to send it to you.
Regards,
Rick

Rams,
Off hand all I can think of is coding the data to a historical average.  I’ll try to think about it a little over the weekend. 
Ross,
I agree with your comments on “Innovative Control Charting”.  Steve is a friend of mine and he always makes light of it when I tell him what a useful book it is.  I’m sure he will enjoy your comment thoug…[Read more]

Rams,
 
I was just wondering if you had considered any Short Run SPC techniques.  Based on the data that you posted you may be able to use normalized control charts or possibly group charts.  The normalized would allow multiple products to be plotted by deviation from target.  In many machining processes this works very well allows cal…[Read more]

Dave,
 
I did read over chapter 9 of “Advanced Topics in SPC”, Wheeler, again.  The method that Wheeler uses takes into account the number of subgroup measured before the shift is detected.  His method would result in higher sample quantities for a k=1 (1 subgroup) detection, but similar sample quantities as the formula I listed at higher k leve…[Read more]

Dave,
I’ll definitely re-read chapter 9 of “Advanced Topics …SPC” by Wheeler.  It’s one of the best SPC books around and even though I’ve been through it repeatedly, I’m sure that I could miss something.
As far as references for the formula you can look up Sample Size for a Two-Tailed Hypothesis Test about a Population Mean in about any…[Read more]

Dave,
You made a few good points, but I think that not being able to post the full formula created some confusion.  In the example you present you have both alpha and beta at 95%, actually they are dependent.  As one increases the other has to decrease.  If beta is 95% then alpha is only 5%.  You also have to use only 1/2 of the alpha in this equ…[Read more]

Lin,.
I can’t get the formula listed here correctly.  If you leave your email I’ll be glad to send it to you or if there is a way to post it here for everyone I’ll be happy to do that.
Rick
 

Yee,
If you don’t mind posting your email I’ll be glad to send you what I have. 
Rick

Rahul,
If you don’t mind leaving your info, I’ll be glad to email or fax some guidelines.  It’s a little complicated getting the calculations listed here.
Rick

Manuel,
Go to
http://www.infinityqs.com/frameSupport-tn.html
and view the Capability document.
Hope this helps, but if you need any more info let me know.
Rick

Good point Jagdish….
Rick

Sample size is critical to the success of control charts.
 
Sensitivity increases as sample size (n) increases.  Two types of error can result from making decisions based on control charts and these errors are dependent on each other and on sample size.
 
Type I Error:   Alpha(a)
 
a risk is the probability of concluding a process contro…[Read more]