I haven't noticed it since I've been back on the Cove (could be here and I just haven't seen it), but you could consider the cost of the NC as the basis for classification.
This is using actual
costs, not what the accounting ppl refer to as "unrealized losses" (like potential lost business) which is what Stijloor points to when he says "how "bad" the customers would
be inconvenienced" (potential vs. actual).
E.g. Job shop operation where a team works on a product for 2 weeks and the customer doesn't accept it.
Root cause analysis shows the major cause being ineffective design validation.
The NC cost will include labour (the correct spelling
for you non-British-language writers), direct materials, other direct costs and overheads.
At the end of the period you can then report: "We had 6 NCs costing over $1 million, 10 NCs between $500K and $750K, and 87 NCs under $500K".
When you add all NCs within a category, you may find the total cost may surpass the total cost of the category above, e.g. 6 x $1.5million = $9 million vs. 10 x $950K = $9.5 million.