It's hard to trust a study that uses a data like this. Even though my situation is anecodotal, sites like Trulia and Zillow have been criticized for their data accuracy in the past.
If you're only willing to trust data sets that are 100% clean/accurate, then you aren't going to trust many data sets.
Is there any reason to believe that Trulia has systematically biased or inaccurate data?
http://www.trulia.com/corp/files/2011/05/trulia_direct_refer... http://info.trulia.com/file.php/5288/Trulia-Estimates-Region...
I've seen appraisals of the same property vary by as much as +/-25% based on which comps they choose and the appraiser's mood that day.
Going into our last housing bubble, appraisers were not estimating true value of a propery but propelling it ever higher because of short supply. Every new sale lifted the prices higher because every appraisal was based on a previously inflated sale price in the area.
ETA: And if your mention of the bubble is to suggest that my home was appraised at a higher value and is now lower to reflect the true value, I purchased my house a year ago when prices of homes were at some of their lowest points.
No, I'm not suggesting your appraiser's valuation is worse than trulia. During the bubble appraisers were valueing homes at whatever the ask was because there was nobody to say any different.
An appraiser is not a highly trained or skilled person. Their value judgement is crucial so it should be a much more involved process than it is with more than one person's opinion reflected.