National data on home prices are sometimes contradictory — should the numbers be factored in to your decision to buy or sell a home?

This post comes from AnnaMaria Andriotis at partner site SmartMoney.

Data released Wednesday morning shows U.S. home prices continue to fall — a stark contrast with a Realtors’ report released last week. Which numbers should home buyers and sellers believe?

S&P/Case-Shiller Home Price Indices reported today that U.S. home prices fell nearly 3% for the three months ending in March compared with a year prior. Nationally, prices hit new post-crisis lows with average home prices in five cities — Atlanta, Chicago, Las Vegas, New York and Portland — reaching a new bottom.

In contrast, the National Association of Realtors‘ data for March showed that median prices for existing-home sales rose 2.5% from a year prior. Last week, NAR released its April report, which showed prices spiked 10%.

Much of the discrepancy comes down to how the two groups measure sales. The NAR tracks existing homes that sell each month, while the S&P/Case-Shiller looks at repeat sales, meaning the price a home recently sold for compared with what the same property fetched its previous sale.

In other words, the S&P is looking at how the price of a specific home changes over time, while the NAR is looking at the prices of homes sold in one month versus previous ones. If more large, expensive homes sell during one month, that will push the median price the NAR tracks up, but it may not mean prices overall are rising.

On top of that, the NAR’s data is monthly while the S&P/Case-Shiller is based on a three-month moving average.

For homebuyers and sellers, the two reports make it difficult to discern if the housing recovery is gaining steam or if home prices will continue to tread along the bottom for some time. “Because of the peculiarities of the housing market, trying to gauge it on a monthly basis is very hard,” says John Vogel, adjunct professor of real estate at Dartmouth’s Tuck School of Business

For homebuyers and sellers, the two reports make it difficult to discern if the housing recovery is gaining steam or if home prices will continue to tread along the bottom for some time. “Because of the peculiarities of the housing market, trying to gauge it on a monthly basis is very hard,” says John Vogel, adjunct professor of real estate at Dartmouth’s Tuck School of Business. (Post continues below.)But experts say the national data impact buyer and seller psychology. For instance, when home prices stay low, fewer sellers are willing to sell their homes on the cheap. As of April, for-sale listings of existing homes were down 22% from a year prior, according to the NAR.

While rising sales have pushed inventory down, housing analysts say listings are dropping because homeowners are choosing to wait until the housing recovery gains momentum and their homes can sell for more money.

National data is less important than local information

The NAR, however, says individual buyers and sellers shouldn’t pay much attention to the national data. “These numbers are meaningless to people making buying and selling decisions in local markets,” says spokesman Walter Molony. Instead, he says, they should be viewed as a broad indicator.

Maureen Maitland, vice president of S&P Indices, says people shouldn’t take one month of data as the story for the year but should look over several months of data.

Rather than tracking national data, home buyers and sellers should focus on data within their local housing market, says Jack McCabe, an independent housing analyst in Deerfield Beach, Fla.

Consumers can find out what prices homes in the neighborhood have been selling for by searching listing sites such as Trulia and Zillow that show recent purchases prices for homes. They can also speak with independent housing experts in their market or with real estate agents.

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For those who choose to pay attention to the national data, experts recommend digging deeper into the reports.  For instance, the NAR tracks home prices for up to 156 metro areas on a quarterly basis. Beyond that, its associations in larger states, including California, New York and Illinois, go further, providing data (on its website) on even more cities and towns.

In high-priced markets in large cities, the S&P Case-Shiller is a helpful starting point, experts say: It offers buyers and sellers data on how higher-priced homes are performing compared with the other price points in the same city.