# Initial Capability study on N50 parts - Subgroup Size?

#### Manix

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HI I am really asleep today and I have come across a bit of a confusing situation.

If I am doing an Initial Capability study on N50 parts. Should my subgroups be 5 (so 10 subgroups) or 10 (so 5 subgroups)?

I am really trying to figure out the logic, I know it's there but seriously I am still asleep.

D

#### D.Scott

Re: Subgroup Size?

Assuming you have already decided to use an x-bar, range chart, you would typically use a subset of 5. You could use fewer or more in each sample depending on what you wanted to find in the process.

I am assuming you are only looking for the simple answer so I would say 10 sets of 5.

Dave

#### Manix

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Re: Subgroup Size?

Yeah Thanks D Scott.

Just had a coffee and woken up a bit. The confusion arose because I have a form that actually arranges the data into 5 subgroups of 10! Which I would almost never use!

I need to change that! I will use 10 subgroups of 5

#### Statistical Steven

##### Statistician
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Your subsgroup size should be based on some rationale of the production process. Deciding you want to sample 50 pieces then looking for a subgroup size is not necessarily representative of the process. If the process generates 3 pieces an hour, then you might select 3 as your subgroup size since it represents an hour of production. In this case, you might have 20 subgroups each have 3 samples. Let your process dictate your subgroup size. I call this concept (as many of call it too) Rational Subgrouping. Since the variability within subgroups and across subgroups has a meaning (Cpk or Ppk), then the selection of subgroup size is critical!

#### Darius

##### Quite Involved in Discussions
with Statistical Steven.
Read first about rational subgroup size, the subgroup size must not be taken without taking in account the "rationale" to subgroup and the size of it.
A wrong subgroup size could make the between sample variation smaller than the within sample variation ( as Donald Wheeler said ). In some industries even subgroup of one could be a good option, without any previous knoledge of the kind of industy and sampling conditions one can not safety say to use 5 (the magical number for SPC practitioners:truce.
Can you elaborate on the kind of industry or process?
How is the sampling, samples per hour,shift,product?

#### Bev D

##### Heretical Statistician
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Steven is "spot on" about the rational subgrouping...

A second angle to ponder is the phrase "initial capability". In the States this often means either a random sample of 50 untis with the capability index calculated using the formula with the total standard deviation; NO subgrouping. This approach is typically used on an established process that has little or no historical data or the data is not used in control charts.

The other approach for an intitial study uses the same formula but the sample is not random and the process is not established - it is brand new and the 50 units are often from the initial running of the process.

In both cases subgrouping is not done.

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