What is subgroup?

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While doing the Statistical Process Control (SPC) study , we have taken the data from the process. As we learn in SPC implementation cycle during the data collection stage for analysis we have to define the strategy for fix the sample size or subgroup size. For further calculation of Process variation the subgroup size is important.

Subgroup is when the multiple observation / reading is collected closed together in time.

Lets take an example: Fig-1, In an auto parts manufacturing company, sample of shaft’s and inspector measure the diameter of shaft’s. For every 2 hour he measures the 5 samples from the production line. Now the data is being capture for calculate the variation in process, so for this condition the subgroup is 5.


Similarly for the larger subgroup size, Fig-2 if we take the oil canning company. They measure the weight of oil cans. The samples of 10 cans after every 30 min. Here the subgroup size is 10 for estimation of process variation.