Defining Subgroups and Importance of Subgroups in SPC and Process Capability

[virpatel]

Hi everyone,

can anyone explain what is subgroup and criteria to select subgroup and its importance in SPC and capability study?

Thanks All.

 

[Stijloor]

Re: Define subgroups, importance of subgroup in SPC and process capability

Hi everyone,

can anyone explain what is subgroup and criteria to select subgroup and its importance in SPC and capability study?

Thanks All.

Hi Virpatel,

For starters, scroll down this page for threads on this subject.

Also, look here:

Rational Subgroups - Samples.

Stijloor.

 

[Bev D]

Stijloor gave you a great start. The question you posted is fairly large in scope. Do you have a more specific question? Or perhaps an example that has you confused?

 

[bobdoering]

This posting and this posting are also a good information.

 

[virpatel]

@stijlor, Thank you very much for providing me a link.

@Bev D, i was running control chart for steam sterilization unit. i took 30 samples, 1 per each hr. The result came out that four or five point are out side of one sigma. i know for my experiment the subgroup size going to be one subgroup but i tried to put the subgroup size 2 and came out there is no out of control condition for two subgroup size. so just curious about the subgroup selection. i wanted to know based on what criteria we should decide subgroup size?

 

[Bev D]

What do you mean by "four or five out of one sigma"?

The link Stijloor provided has discussions on rational subgrouping which is how you know how to initially select your subgroups. Subgroups must selected before the product is run not after the fact when you see the data.

 

[virpatel]

@ Bev D, Thank You for your reply. i meant that four of five points are out of one sigma limit.

By the way one more question, after sterilization process i take one sample per hour and measure it for micro test two times.

In another scenario, if i take two samples every hour and measure it for micro one time each.

Is subgroup size 2 in both scenario?