A Comparative Market Analysis (CMA) takes in many factors which Zillow does not. Zillow uses public records to cross reference features like number of bedrooms, square footage, garage?, etc. These public records may not be up to date. Any home and neighborhood that has not been physically examined cannot be accurately valued. This is the only way to determine how busy street traffic is, where are the schools located, etc. Zillow is not totally useless, however. But Zillow data must be cross checked against other data sourses to gain a complete picture of a neighborhool.