Capabilty & samples - Why do we use a d2 (of 2.326) for a sample of 5 parts

Jim Wynne

Staff member
Admin
#11
Star18 said:
I see Steven !
But I'm a little confused about what you said : the only use of the individual values and moving range charts in the case of one part by sample. Our SPC software gives us the possibility of plotting X bar/R charts (and other control charts) if we have a part per sample with grouping them per 5. As a consequence d2 is equal to 2.326.

Someone can comment ?

Rach.
If you're subgrouping the samples after the fact (i.e., you're not collecting them in groups as production is running) your results will be unreliable. Time is an important factor in Xbar/R charting. If you subgroup parts that were produced at significantly different times, you're likely to get wacky results.
 
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Statistical Steven

Statistician
Staff member
Super Moderator
#12
Star18 said:
I see Steven !
But I'm a little confused about what you said : the only use of the individual values and moving range charts in the case of one part by sample. Our SPC software gives us the possibility of plotting X bar/R charts (and other control charts) if we have a part per sample with grouping them per 5. As a consequence d2 is equal to 2.326.

Someone can comment ?

Rach.
Correct, for every subgroup size there is a d2 value that given the distribution of the range is an estimate of standard deviation. Another way of saying it is that there is a distribution for the ratio of rbar to the standard deviation, and this ratio is d2.
 
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