I would say they should not be "deciding" such things - they should be extracting and supporting them from data and analysis.
The problem is, from someone who has good experience at this, is that the data is not good enough.
You would think appraisal organizations would be working with MLS's and other data providers throughout the country to make necessary changes so that residential appraisers could indeed objectively appraise based on data.
But they are pretty much useless to do anything but create more stupid regulations or make minute changes to existing ones, year after year.
FNMA, Freddie Mac, et al, are just as useless.
What is missing for example:
1. For purchases, there is invariably an appraisal with a Cost Approach and Effective Age. That effective age should be entered in the MLS data. And as inaccurate as it may be, it is definitely better than saying a 120 year old home is 120 years - which is what happens if you are running regression on 100 plus properties, - unless you take the time to look at the MLS photos on all 100+ properties and judge effective age yourself. And, quite frankly no one is willing to pay for that.
2. Actual Age is often inaccurate when there have been major additions to homes. The actual age should be a GLA (or as the case may be GBA) weighted value based on the year of construction..
3. Unfinished and basement areas are often inaccurate.
There should be an ongoing project to correct all sales data going back at least 3 years. Corrective data could be put into a separate database, keyed with the same MLS and parcel numbers.
FNMA, Freddie Mac and the VA and other appraisal organizations should pay for these corrections, since they have so far failed in their implicit responsibility to manage these things over the past decades.