The Case-Shiller Index posted awful numbers in its most recent reading. Each of the index’s 20 tracked markets showed home price deterioration between September’s and October’s respective report. Some markets fell as much as 2.9 percent.

The drop in values is nothing about which to panic, however. The Case-Shiller Index is just re-reporting what we already knew. It’s a common theme with the Case-Shiller Index, actually; a trait traced to the report’s methodology.

The Case-Shiller Index is an imperfect housing indicator with 3 inherent flaws.

The first flaw is that the index makes use of a limited data set, tracking values in just 20 cities nationwide. That data set is then projected across the more than 3,100 other municipalities in the United States. The “national figures”, therefore, aren’t really national.

The second flaw is that, even within the tracked 20 cities, not all home sales are included. The Case-Shiller Index only tracks sales of single-family, detached homes, and within that market subset, it only uses homes that are “repeat sales”. This specifically excludes sales of condominiums and multi-family homes, and new construction.

Lastly, Case-Shiller Index’s third flaw is its “age”. The Case-Shiller Index reports on a 60-day delay, and the values it reports are tied to contracts written even longer ago.  Sales contracts from July and August are responsible for October’s closings so when we see the Case-Shiller Index as reported in December, some of the data it’s reporting is 5 months old already. That’s too old to be relevant.

Looking back at 2010, housing was at its weakest between May and August. Therefore, it’s no surprise that the most recent Case-Shiller Index shows significant weakness.  Looking forward, we should expect the report to improve — especially because of how strong New Home Sales and Existing Home Sales have been since summer.

The Case-Shiller Index is helpful for economists and policy-makers. It’s not much good for individual homeowners, however. For accurate, real-time housing data, talk to a real estate professional instead.

According to the Standard & Poors Case-Shiller Index, home values rose 5 percent in June versus the month prior, and 4 percent from a year earlier.  It’s the 16th consecutive month in which Case-Shiller reported an increase in home values and the third straight month of outstanding results.

That said, homeowners and home buyers in Charlotte would do well to temper Case-Shiller enthusiasm. The June figures are issued on 60-day delay and, over the last 60 days, housing data has been lackluster at best.

• Existing Home Sales are down 27 percent
• New Home Sales are down 12 percent
• Homebuilder confidence is down

Stories like these highlight a key weakness of the Case-Shiller Index — it’s out of date as soon as it’s published. Because of this, the Case-Shiller Index relevance to everyday Americans is muted. People don’t buy homes in the “60 days ago” real estate market, after all.

June is ancient real estate history.

However, the Case-Shiller Index does have its place. As the most widely-followed, private-sector housing tracker, the index is used to help make policy decisions and to shape Wall Street’s expectations of the economy. This means that a strong Case-Shiller reading can cause mortgage rates to rise, and a weak Case-Shiller reading can cause rates to fall.

Tuesday, mortgage rates fell.

Standard & Poors released its Case-Shiller Index Tuesday.  The index is a monthly home valuation report from select cities and among the private sector’s most popular home pricing models.

In reviewing the April Case-Shiller Index and its accompanying analysis, it appears that the housing market’s rebound is gathering momentum.

In the index’s 20 tracked cities:

• 18 of 20 improved from March to April 2010
• Versus April 2009, home prices are up nearly 4 percent
• The two “down” cities from April — Miami and New York — are off just 0.5% and 1.0% annually, respectively

Furthermore, as another sign of strength, San Diego, a city in which homeowners have lost a lot of equity since 2007, has now shown 12 straight months of home price improvement.

However, the Case-Shiller Index must be kept in context. It’s far from perfect.

For one, the index reports on a 60-day delay; it’s only now showing data from the end of April, when the federal homebuyer tax credit was expiring. Home sales have been weak since then it’s been reported.

And second, the Case-Shiller Index is limited to just 20 cities nationwide. Therefore, the index doesn’t consider every home sale in every American city — it only considers a select few. Many more U.S. homes are excluded from the Case-Shiller Index than are included.

But, despite its flaws, the Case-Shiller Index remains important with respect to economic analysis. Much like the government’s Home Price Index, Case-Shiller helps to identify broader trends in housing that shape government and monetary policy.

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.