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Shewhart Control Chart for SPC In Excel - Explain the yellow and red marked column

M

Muhd Firdaus

#1
Dear member,

i have found the attached information while browsing internet today and would like to share it with everybody in this forum. However,i would like to seek your cooperation as to explain the yellow and red marked column in the attachment. i do not understand how the value been obtained. Maybe anyone here can explain. Thank You.
 

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DRAMMAN

#2
Re: Shewhart Control Chart for SPC (using excel)

If you are asking what an "A2" constant is I will attempt to explain it and where it came from. The typical control limit formula is (X double bar)+-3std deviations. If you have ever manually calculated the standard deviation of a large data set you understand it is time consuming and relatively complicated. You have to remember that SPC was invented in the 1920's so the average person did not have any kind of advanced math training and there were no computers or calculators. Recognizing this, some smart people came up with these constant's to simplify the calculation of the control limits. These days, the modern SPC computer application gives you the ability to decide between using the constants or using the actual std deviation. The A2 constant changes depending on your sub-group size. A 5pc sub-group has a different A2 than a 4pc sub-group and so on. If you have only used SPC in the computerized age or never did SPC on paper charts then it would be normal to have no idea what A2 is. So essentially, the document describes how to create SPC charts in Excel replicating the original manual paper based approach. You could also make SPC charts in Excel using the actual STD Dev calculations but it is a little easier to construct the file using constants. Both approaches are acceptable and will usually yield only slightly different CL's. The only way to truly understand this is to gain a solid complete understand of how to construct SPC charts manually on paper. Many of us have created our own SPC Excel files. You could probably find a 100 examples on this forum.

My advice is to study the SPC basics first before trying to create Excel files to make SPC charts.
 

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#3
Re: Shewhart Control Chart for SPC In Excel - Explain the yellow and red marked colum

Note - tables of the A2 value versus subgroup size may be found in several stats textbooks (including Acheson Duncan's Quality Control and Industrial Statistics) or google "Control Charts Constants". This link http://www.buec.udel.edu/kherh/table_of_control_chart_constants.pdf is the first one that came up for me.

If you generally stick to the same subgroup size, such as 4 or 5, then you don't have to keep searching the table. That is, unless you have some rational need to change subgroup size.
 
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