Cook County’s Tax Algorithm Worked As Designed. That Was The Problem. – Forbes

"If the model is fair, why did bills go haywire? The answer sits one floor down, at the Cook County Board of Review, a separate elected body that hears appeals after the assessor sets a value. The assessor proposes; the Board disposes. And the Board has been busy. The University of Chicago report found that commercial appeals there shift 3-4 percent of the tax base onto residential property every single year, effectively undoing the equity the assessor's office tries to build in."