In a letter to its seller-servicers last week, Fannie Mae updated its appraisal policies to provide specific guidance on when an appraisal is considered deficient. The policy change was prompted by Fannie Mae's post-purchase reviews, which turned up cases where lenders had reduced the opinion of market value in the appraisal report based upon underwriter judgment, automated valuation models or another methodology.
https://www.efanniemae.com/sf/guides/ssg/annltrs/pdf/2010/sel1009.pdf