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X BAR - RANGE chart - Subgroup question

Former Member
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From the xMII 11.5 help on SPC charts -

http://help.sap.com/saphelp_xmii115/helpdata/en/Advanced_Topics/Illuminator_Quality_Portal/iSPCChart...

SaleAmount OrderID CompanyName ShippedDate

2556.95 10393 Save-a-lot Markets 01/03/1997 00:00:00

2505.60 10398 Save-a-lot Markets 01/09/1997 00:00:00

3868.60 10401 Rattlesnake Canyon Grocery 01/10/1997 00:00:00

2713.50 10402 Ernst Handel 01/10/1997 00:00:00

1) If I were plotting the saleamount, what are the considerations for choosing a subgroup size for plotting a "X Bar - Range" chart for the ABOVE sample data?

2) If I were to select a subgroup size = 3, (and therefore consider the first 3 items as one subgroup) would this be a correct method of selecting the subgroup size, since the variation within the subgroup ie. between item 3 (Saleamount = 3868) and items 1 and 2 (Saleamount approx. 2500) is large?

Appreciate your ideas!

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Answers (1)

Former Member
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That's a really bad example of a dataset or application for SPC...but, in general, subgroups should share some common characteristics and should be determined prior to data collection, as the sampling plan is a key aspect of the statistical control approach.

Former Member
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You are right. I thought so too.

I found out that there may not be any definite answers to my question and subrouping should ideally be done on the principle of "Rational subgrouping"

Also, greater the subgroup size, the better - since the average of the subgroups would tend to form a Normal distribution.

Please add if you have any other thoughts

Thanks.