Last week, Standard & Poor’s released its Case-Shiller Index for December 2010. The index is a home valuation tracker, meant to meausure the change in home prices from one period to the next.

December’s Case-Shiller Index showed major devaluations nationwide. As compared to December 2009, on a year-over-year basis, home values fell in 18 of the Case Shiller Index’s 20 tracked markets, and the U.S. National Index dropped 4 percent overall. 

The retreat puts December’s home values at similar levels as compared to early-2003.

That said, buyers and sellers in the King County area would be wise to take the findings lightly. The Case-Shiller Index is inherently flawed. As such, its results are neither practical — nor relevant — to everyday Americans.

There are 3 Case-Shiller flaws, in fact.

The first flaw is the index’s limited sample set. Wikipedia lists 3,100+ municipalities nationwide and we can be certain that real estate is bought and sold in all of them. The Case-Shiller Index, however, measures just 20 of them. That’s less than 1% of all U.S. cities. And then, within those tracked cities, Case-Shiller reports an average, lumping disparate neighborhoods and streets into one big number.

The “national figures” aren’t really national, and the “city data” doesn’t apply to your home, specifically.

The second Case-Shiller Index flaw is how it measures home value changes. The index only consider at “repeat sales” of the same home, so long as that home is a single-family, detached property. Condominiums, multi-family homes, and new construction are ignored in the Case-Shiller Index.

Because distressed properties account for such a high percentage of resales lately — 36% in December –foreclosures and short sales skew Case-Shiller Index worse.

And, lastly, the Case-Shiller Index is flawed by “age”. Because it reports closed sales a 60-day delay, December’s Case-Shiller Index is measuring the values of home sales contracts from September and October. The Case-Shiller Index, therefore, is a snapshot of the not-so-recent past, and does little to tell us about the next 60 days.

Overall, the Case-Shiller Index is helpful tool for economists and policy-makers, but it doesn’t do much good for individual homeowners across the city of Olympia or anywhere else. For accurate, real-time housing data in your local market, talk to a real estate professional instead.

Using data compiled in December, Standard & Poors released its Case-Shiller Index Tuesday.  The report shows home prices down just 2.5% on an annual basis, a figure much lower than the 8.7% annual drop reported after Q3.

According to Case-Shiller representatives, the housing market is “in better shape than it was this time last year”, but some of the summer’s momentum has been lost. 15 of 20 tracked markets declined in value between November and December 2009.

Meanwhile, it’s interesting to note the 5 markets that didn’t decline — Detroit, Los Angeles, Las Vegas, Phoenix and San Diego.  Each of these metro regions were among the hardest hit nationwide when home prices first broke.  Now, they’re leading the pack in price recovery.

For some real estate investors, that’s a positive signal.  But we also have to consider the Case-Shiller Index’s flaws because they’re big ones.

As examples:

1. Case-Shiller data is reported on a 2-month lag
2. The Case-Shiller sample set includes just 20 U.S. cities
3. There’s no “national real estate market” — real estate is local

That said, the Case-Shiller Index is still important. As the most widely-used private sector housing index, Case-Shiller helps to identify broader housing trends and many people believe housing is a key element in the economic recovery.

If the markets that led the housing decline will lead the housing resurgence, December’s data shows that full recovery is right around the corner.