That is because the minimum number of parts depends on the situation.
Statistical Steven is correct. there is NO universal minimum size.
As described in my paper, it all depends on what you are trying to do or understand and what defect rate you have and the distribution of that defect rate. There are statistical considerations as Steven pointed and non-statistical considerations.
An additional statistical consideration, as Steven alluded to, is the amount of disagreement you get. This is one of those iterative situations, you need the data to tell you how much data you will eventually need...the whole idea is to understand the limitations of your system, then improve it and test again. If you get 100% agreement there is no problem with a reasonably large enough sample (say 50 parts)
A non-statistical consideration is how many marginal units are in normal production and how much time the inspector or machine has to perform the inspection. If the defect rate is fairly low, there a decent proportion of parts that are marginal (both passign and failing) and the tiem to inspect is rather short a much larger sample size is necessarry to test teh actual systme capability. It's easier to determine pass/fail on marginal units when one isn't under time pressures so you get a better score, but in real life - where the capability of the system actually matters - it's very difficult to get it right. I have encountered this situation many times and have ended up testing 400-1000 parts on the line to mimic actual conditions.
in order to give you good advice we need to understand:
- what are trying to accomplish? (e.g. 'Check the box' to submit a study to your customer or do you want to learn about your system and improve it if needed)
- What is your measurement system?
- What type(s) of defect(s) are you trying to detect?
- what is the defect rate?
- What is the distribution of marginal units?
- What is the inspection environment? (e.g. quiet with plenty of time or inline high speed high volume?)
If you are really are just trying to 'check the box' (and many people are) then my answer would be to do 50 parts, 25 bad and 25 good. but others will have different answers because there is no single minimum size.