Standard & Poor’s released its September 2011 Case-Shiller Index this week. The index tracks home price changes in select cities between months, quarters, and years.

The Case-Shiller Index for September showed drastic devaluations nationwide.

As compared to August, home values fell throughout 17 of the index’s 20 tracked markets, led by Atlanta’s 5.9% drop. On an annual basis, home values have now returned to early-2003 levels.

That said, home buyers and sellers should be cautious when referencing the Case-Shiller Index. The index is a flawed metric and, as such, can lead to improper conclusions about the housing market overall.

The Case-Shiller Index’s first flaw is its most obvious — its limited sample set. 

According to Wikipedia, there are more than 3,100 municipalities nationwide. Yet, the Case-Shiller Index includes data from just 20 of them in its findings. These 20 cities account for fewer than 1% of all U.S. cities, and just a small percentage of the overall U.S. population. 

The “national figures” aren’t really national, in other words.

Even on a city-by-city basis, the Case-Shiller Index gets it wrong.

By lumping disparate neighborhoods into a single, city-wide result, the index ignores the relative strength of one area at the expense of another. In the aforementioned Atlanta, there are areas that fared much better than September’s -5.9% as cited by Case-Shiller. Some areas fared much worse.

A second flaw in the Case-Shiller Index is it’s methodology for measuring changes in home value. The index only considers “repeat sales” of the same home in its findings, and those homes must be single-family, detached property. Condominiums, multi-family homes, and new construction are not included.

In some cities — Chicago, for example — “excluded” property types can account for a large percentage of total monthly sales.

And, third, the Case-Shiller Index is flawed by “age”.

Because Standard & Poor’s publishes on a 60-day delay, the Case-Shiller Index is reporting on a housing that no longer exists. Sales that closed in September are based on contracts written from June-August –a time-frame that’s 6 months aged.

The best use of the Case-Shiller Index is as an analysis tool for economists and policy-makers interested in the long-term trends of U.S. housing. The index does very little good for every day buyers and sellers, unfortunately.

For up-to-date, accurate market data, talk to a real estate professional instead.

The August 2011 Case-Shiller Index was released this week. On an monthly basis, 10 of 20 tracked markets worsened. On an annual basis, valuation degradation was worse.

Only Detroit and Washington, D.C. posted higher home values in August 2011 as compared to August 2010, rising 2.7% and 0.3%, respectively.

However, the index has been moving in the right direction. Since bottoming out in March of this year, the Case-Shiller Index is up nearly 4 percent.

As home buyers and sellers , though, we have to remember that the Case-Shiller Index is a flawed product; its methodology too narrow to be the final word for housing markets.

The Case-Shiller Index has 3 main flaws.

The first Case-Shiller Index flaw is its relatively small sample size. Although it’s positioned as a national housing index, Case-Shiller data represents just 20 cities nationwide, and they’re not even the 20 most populous U.S. cities. For example, cities like Houston (#4), Philadelphia (#5), San Antonio (#7) and San Jose (#10) are excluded from the Case-Shiller Index findings.

By contrast, Minneapolis (#48) and Tampa (#55) make the list.

A second Case-Shiller Index flaw is the way in which it measures home price changes. The Case-Shiller Index formula ignores all home sales except for “repeat sales” of the same home. New homes don’t count for the Case-Shiller Index. Furthermore, the index ignores condominium and multi-family home sales, too. 

In some cities, condos can account for a large percentage of sales.

And the third Case-Shiller Index flaw is that the data is reported on a 2-month lag. Next week marks the start of November, yet we’re still discussing data from August. A lot can change in two months (and it often does). Today’s market conditions are similar to — but not the same as — market conditions from before Labor Day.

The Case-Shiller Index is far from “real-time”.

As a monthly release, the Case-Shiller Index does more to help people with a long-term view of housing, including politicians and economists, than it does for everyday buyers and sellers who negotiate prices based on current demand and supply.

A real estate agent can tell you which homes have sold in the last 7 days, and at what prices. The Case-Shiller Index cannot.