I-mr VS X-mr Chart - What's their difference?

[Miner]

The X or individuals chart is ALWAYS one single data point plotted on the chart. The n number comes from how many consecutive points are used to calculate the Moving Range.

In other word, the x chart is plotted using individual points (n = 1) while the MR chart is plotted using subgroups of n = 2 or more.

[AdamP]

One thing that made a lot of sense to me when being taught the IMR and other control charts was that when you look at subgrouped data - say on an X-bar/R chart, the questions you've asked of your process are these:
What's changing between subgroups? (differences in the X-bar chart)
What's NOT changing between the subgroups?
What's changing within the subgroups? (look at the R chart here)
What's NOT changing within the subgroups?

So it's a "within v between"discussion on process and sampling variation.

When you don't have subgrouped data and are looking at the IMR charts, the questions shift a little and you're now asking about longer term variation on the I or X chart and about short term variation on the MR chart. The reason you can select n=2, n=3 n=6, etc for the MR chart is because you need to define what short term really is for your process.

If you're in an industrial setting and looking at an assembly process, n=2 is pretty common. We used to look at banking center activity where the feature of interest might happen every 5th day, so we set n=5 for our charts - just as an example.

Hope it helps!

Cheers,

Adam

[Darius]

In Reply to Parent Post by Miner

This is a link to an article from George Mason University and table for I-MR constants for moving ranges of varying sizes.

Sorry but I don't understand, although the note "Typically we look at two consecutive periods" appear, there is no other mention to ImR on several periods.


The table is E2 constant = 3/d2 = 3*A2*(subgroup_Size)^0.5, although found the table
http://world-class-quality.com/image...20Formulae.pdf

Can't find anything about that

Is there on the WEB an example of such method?

[Sitiz]

Yes Darius,I share the same understanding with u..I cant find example where more than 2 subgroup is used in IMR case.

[Miner]

Darius,
In both the linked article and in the table to which you linked there is a column entitled E2. This is shown for sample sizes greater than or equal to 2. E2 is the control limit constant based on the moving range and may be used for more than 2 consecutive points.

Sitiz,
I have not seen examples where more than 2 are used either, but can see situations where it would be appropriate. One example would be in an autocorrelated process where true process variation occurs over a longer period of time. In this case, you would need to include enough consecutive points to pick up this variation.