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MLS? That's the obvious answer... But why is there differences in the houses that are shown as available on sites like Movoto.com, RedFin.com, and Zillow.com?

Can anyone describe the discrepancy? And which is the most accurate (if there is a non-subjective answer to that)?

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a answer for the UK would also be of interest – DoNotInstall Jul 21 '10 at 9:04
@DoNotInstall - the UK is a totally different ballgame, individual agents pay to have properties listed on one or many "aggregate" sites. Each aggregate site will have listings from a different combination of agents and the method agents use to "upload" a listing will be different for each aggregate site. – Sohnee Aug 4 '10 at 12:23

In the United States, Redfin is organized/licensed as a real estate brokerage and hence has access to MLS (multiple listing service) data. Movoto's home page says they're a full service brokerage - I haven't used their site and don't know much more about them, but I imagine they've got the same MLS data.

Zillow is not a brokerage and does not have access to MLS data, they've developed their own database from other sources.

Typically, to get access to the MLS data for a given state or region, you need to be licensed as a real estate broker/agent or as an appraiser for that state/region.

In the real estate industry, the term for a website that includes MLS data is "virtual office website" or VOW. There has been quite a bit of fighting and arguing and drama within the real estate industry about sharing data and setups which allow Broker A to build a website which shows Broker B's listings, even though in bricks-and-mortar world, Broker B shows Broker A's houses all of the time, and that's what MLS was set up to facilitate.

Another source of data is public records - typically, when property changes hands, a deed or other document is recorded in the county recorder's office. The deed is public record, and public record companies copy, index, and resell that data. In California, it's usually possible to back-calculate the sale price by looking at the documentary transfer tax paid to record the deed, so transaction prices are for the most part public records, too, as are mortgage amounts. This is where Zillow gets their data about historical transactions and trends.

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If many sites are pulling data from MLS, it is highly possible that they operate different strategies for obtaining the data. For example, they might refresh their listings every hour, or more or less frequently.

Also, they may have criteria for whether a listing is displayed. If a piece of information is missing, they may exclude it from being show.

Another cause for differences may be if they weight their searches in a different way - the result set may contain the same listings, but in a different order.

It's hard to accurately answer the reasons for the differences, but I hope that these pointers are useful.

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