Bracketing comparables as expressed by HUD
Comparable Selection
At a minimum, comparable selection should be based on properties having the same or similar locational characteristics as well as physical characteristics which includes: style, age, size, utility and condition.
Comparable sales should never be selected based on sales price.
In selecting comparables, use the bracketing method. Bracketing, as defined in The Dictionary of Real Estate Appraisal, Fourth Edition, Appraisal Institute, is “a process in which the an appraiser determines a probable range of values for a property by applying qualitative techniques of comparative analysis to a group of comparable sales. The array of comparable sales may be divided into two groups – those superior to the subject and those inferior to the subject. The adjusted sales prices reflected by these two groups limit the probable range of value for the subject and identify a bracket in which the final value opinion will fall.” It is advisable to bracket sales using both dwelling size and sales price whenever possible. If bracketing is not possible, the appraiser should explain why.
http://www.HUD.gov/offices/adm/hudclips/handbooks/hsgh/4150.2/41502appdHSGH.doc
I don't make the rules I just keep explaining why they don't work.