K
klluk
we plotted individual x chart but my boss is asking the UCL=barX+3sigma, how comes the equation is barX+2.66 bar(MR) is used? Is it related to the variation, pls help to explain
Re: individual x chart
well a simply reply to him would be:
3*sigma is a measure of the population variation, but it is an unknown quantity which can only be estimated by sampling. For this case we used moving range, which needs a correction factor for a more accurate estimate.
Thus, 2.66.
well a simply reply to him would be:
3*sigma is a measure of the population variation, but it is an unknown quantity which can only be estimated by sampling. For this case we used moving range, which needs a correction factor for a more accurate estimate.
Thus, 2.66.
D
Darius
sigma = MR/d2
d2(2) = 1.128
UCL=barX+3sigma = barX+3* MR /d2 = barX+3/d2 MR = barX+2.66 MR
d2 table was built to determine the economical control limits by Shewhart in the 20's .. 30's working for Bell labs.
P
pagnonig
As Dr. Wheeler explained very nicely in his books, sigma is a global measure of dispersion which does not take into account the time behaviour of the data and its "between subgroup" variation, even if we are talking about subgroups of 1 in individual charts.
2.66 * MR consider the data MR and therefore is dependent upon data time distribution.
This is the reason why, if you use limits caluclated with 3sigma, you run the risk to catch the wrong signals (or miss the right ones) coming for your process.
The attachment may help you to clarify this subject.
Ciao!
Giuseppe
2.66 * MR consider the data MR and therefore is dependent upon data time distribution.
This is the reason why, if you use limits caluclated with 3sigma, you run the risk to catch the wrong signals (or miss the right ones) coming for your process.
The attachment may help you to clarify this subject.
Ciao!
Giuseppe
I like the conclusion of his article:
"Many articles in technical journals and many software packages in use today
actually use the wrong methods. As to why this is the case, I can only assume it is because novices have been teaching neophytes for so many years that the teaching of SPC is out of control."
Another point on which I agree with Dr. Wheeler. I see it all the time in precision machining.
It's the part about having to "think" and not just rubber stamp charts that makes it so hard for people - especially when they have been trained to rubber stamp.
