I gave a presentation to the Saint Louis Chapter of the American Society of Appraisers on June 14th. You can find the full deck of slides here, (click on first slide, then use arrow keys to advance slides) but the graphic below is the main point.
The graphic reports the difference in logs of the County appraisal value of single family housing and the log of the sales price of each house after the official county appraisal. The red line uses only arms-length house sales (County validity code X) and the green line uses only distressed house sales (County validity codes 5 and Z).
Prior to 2007 Saint Louis County routinely under-appraised properties by 20 percent or more. From 2007 to 2013 the County basically hit their mark, and are well within the legal tolerance set by the State. I will update by model to include all of 2015.