Compare and adjust according to market data. To give every appraiser license to go out and make up definitions/ standards of measurement and reporting as they go along, using only "the market" as a source is a dangerous (and ridiculous) notion. It just seems counterproductive for everybody to be acting/writing their own glossaries, standards and methods when there are plenty out there to guide us. Today it's GLA, tomorrow it's something else. If the relativist madness continues we'll soon have to read a two page definition of every line on the form just to have any clue what the appraiser is doing or how things are defined.
There are EXTREME cases where it might be warranted, a few related by others here on the forum (Barry Dayton has had at least a doozie), but this work-a-day stuff is pretty elementary. Why re-invent the wheel everytime we have something a little bit different?