I’ve done various ‘quality engineering’ tasks in my career. A very high confidence level for a particular variable can be obtained by measuring that variable on 30 sample parts that are all produced using the exact same process, materials, tools, etc… However, the level of confidence needed (acceptance criteria) is established based on a lot of inputs (anticipated production/sales volume, acceptable deviation from nominal, severity of non-conformance, etc…). Not all brands will come up with the same acceptance criteria based on their business model.
Something to keep in mind is that any variable (dimension, material property, etc) that could result in a failure, like a shift in zero or tracking variance, has to be collected and monitored in order to ensure conformance. Usually, this is multiple characteristics on each individual part. In the case of a scope there would be multiple parts that could give similar failure modes. I note that to say that for lower volume brands, it may well be less costly to fix as fail, based on the customer’s feedback, rather than invest in the resources to develop detailed quality control plans and maintain them.
