Per MSA manual, 4th edition, Stability study is to be condcuted by drawing average and range charts and points beyond UCL and LCL will indicate that measurement system is sable or not. While using control charts for monitoring of production process, general practise is to first calculate trial control limits and then use them to determine presence of any special cause.
My questiuon here is how trial control limits will be calculated for stability?
Apologies to all those who were citing my name for input. I was traveling over the weekend and was not checking in here.
@Rameshwar25 I am not entirely sure what you are asking. Are you asking about the formula used to calculate limits or the minimum number of samples? Let's start by using the concept of phase 1 and phase 2 control charts.
- Phase 1 - The formula used would be the same formula as used for phase 2 control charts. Phase 1 is mainly investigatory and consists of using temporary control limits to determine whether the process is stable enough to calculate more permanent control limits. The phase 1 control limits are only temporary in the sense that the smaller number of subgroups used gives more uncertainty about actual control limit values. However, as you collect the data from more subgroups and calculate phase 2 control limits the actual difference will tend to be quite small UNLESS the process was unstable to begin with.
- Phase 2 - Again, the formula used to calculate phase 2 control limits is the same formula as used in phase 1. The difference is that you have collected more data/subgroups and have higher confidence that the process is stable. Whereas. phase 1 was almost entirely composed of short-term variation, phase 2 has a greater opportunity to see the effects of long-term variation.
Now if your question pertains to the minimum number of samples required, AIAG would be consistent with the requirements shown in the SPC manual, namely 100 data points, which they tend to state as 20 subgroups of 5. However, Dr. Wheeler would argue that you can calculate reasonable phase 1 control limits with a much smaller number of subgroups. You can search my posts on this forum as I have referenced this several times.
One last note: In most cases, a stability study is not necessary unless you are making measurements that will be affected by changes in temperature (i.e., precision machining), or by warmup of electronic equipment (rare with newer equipment). I recommend performing a risk analysis first to determine whether a stability study is even necessary.
If I missed the intent of your post, please clarify it and I will respond more promptly now that I am back.