Typically there are tolerance levels that users of appraisals prefer in terms of gross adjustments to comparables and net adjustments made to comparables. I don't see any mention of those kinds of standards. That was the intent of my search here in the first place. Maybe someone could add this to the page. 

 Also recently there has been much effort by the mortgage lending industry to identify best practices for appraisals and quality assurrance in the use of appraisals. This work resulted from the major real estate market correction, MBS/CMO debacle and broader recession.