Here's my thought on this. I believe the 8/23/17 interagency HMDA Examiner Transaction Testing Guidelines still apply to all data fields, even though the guidance on the key data fields is more lenient than the 8/23/17 guidance. In the guidance for key data fields (for example, FDIC FIL 51-2017), the agencies still make disclaimers that non-key fields could be reviewed as appropriate. For example, FIL 51-2017 says this:"However, in certain circumstances, consistent with the FFIEC Guidelines, such examination staff may determine that it is necessary to review additional HMDA data fields, as appropriate."
In addition, the March 2019 OCC guidance David cited also says this:"In certain circumstances, however, and consistent with the FFIEC guidelines, examination staff may determine that it is appropriate to review additional HMDA data fields[/b]."
To me, this says that you can't focus solely on the key fields and just ignore the other fields. So if I were conducting internal testing, I would want to be as clean as possible and, therefore, would probably utilize the 8/23/17 guidance without (much) regard to the key fields guidance. In other words, I wouldn't just test the "key fields" but would evaluate my LAR - and possible voluntary resubmission requirements - based on the original 8/23/17 guidelines. The idea is that if you evaluate yourself under the stricter guidelines, you should be sufficiently prepared in the event that examiners use the discretion outlined in the key field guidance. That said, your primary efforts
should go to ensuring the key fields are as clean as possible.
To answer your specific question:
How would you evaluate or communicate an overall error to management or the business units when multiple fields may be involved not just an overall rate?
I would recommend communicating this by following the 8/23/17 guidance and explaining the standards outlined there and how your results compared with this. You could then split out the performance in key fields and non-key fields and make a recommendation (for resubmission) based on performance/results of testing.
I'm sure you are well aware of the 8/23/17 guidance, but if you are looking for a summary of it, here is an article I wrote that summarizes that guidance: https://www.compliancecohort.com/blog/what-is-an-acceptable-hmda-error-rate