It sounds like it could be a questionable finding.

Statistical techniques can be as simple as ratios but the ratios must be defined. CpK, DPM, CPM are all defined ratios that are acceptable. But you have to be able document how the ratio is calculated. For example next years complaint rate is a 15% reduction in Complaints per Million, CPM - then define CPM as number of complaints / total number of products shipped (Last month 1 year ago to last month) or (2019 total shipped units) or (2020 total shipped units thus far) its up to you to determine which number to use depending on your product shelf life, sales volume increasing , decreasing etc .... *(not suggesting CPM is a good metric, most regulatory bodies do not like it since it is in the public health interest to increase the numerator in reporting.)*

As you can see here you can end up defining CPM very differently if you change how you define the denominator. So a pure ratio for example could be non conforming if the method to determine the numerator or denominator is not defined. At a global company with multiple sites the QMS would need to do this or every site may have a varying definition of the numerator and denominator for purposes of reporting.