These sources use what is generally referred to as an Automated Valuation Model (AVM).  An AVM is a mathematical tool that delivers a probable price calculation often inferred to be a opinion.  An appraisal is a process where a person arrives at a value opinion by applying his or her judgment and experience to all aspects of research & development.  An appraisal is defined as an opinion of value, which is distinctly different from a calculation.  An AVM uses automated processes and cannot produce an opinion of value because only individuals can exercise judgment and form opinions.

A few Key Differences:

• IL law requires an  appraiser to identify the relevant features affecting value when developing their opinion. No licensed local experts of any kind are involved in development of the automated algorithms used by these services so no judgment is made regarding what features may or may not affect value in a particular market.
• An appraiser will identify, compare and rate subjective factors such as location, condition & quality when developing their opinion. No licensed local experts of any kind are involved in development of the algorithms used by these automated services so they make no consideration or adjustment for these factors.
• An appraiser is a local research expert that is required by law to verify information and make judgments about conflicting data. No licensed local experts of any kind are involved in selecting or verifying data used in the development of the mathematical models produced by these automated services.
• An appraiser is required by law to disclose their scope of work and make multiple certifications about the process and their objectivity. Every aspect of the appraisal process is regulated buy rule and law to promote transparency and ensure public trust.  Instant value services like Zillow, Trulia and Redfin continue to be immune from law and accountability for the calculations they provide in Illinois and other states.  See dismissed US District Court case No. 17-CV-4008. They continually market and imply that their calculations are legitimate estimates of value that can be used in place of long established valuation methods.