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Quick take: The S&P 500 Index price is 49% above its long-term trend, down from 50% above trend last month.

About the only certainty in the stock market is that, over the long haul, over performance turns into under performance and vice versa. Is there a pattern to this movement? Let's apply some simple regression analysis (see footnote below) to the question.

Below is a chart of the S&P Composite stretching back to 1871 based on the real (inflation-adjusted) monthly average of daily closes. I've using a semi-log scale to equalize vertical distances for the same percentage change regardless of the index price range.

The regression trendline drawn through the data clarifies the secular pattern of variance from the trend — those multi-year periods when the market trades above and below trend. That regression slope, incidentally, represents an annualized growth rate of 1.73%.

(click image to enlarge)

The peak in 2000 marked an unprecedented 155% overshooting of the trend — nearly double the overshoot in 1929. The index had been above trend for two decades, with one exception: It dipped about 10% below trend briefly in March of 2009. But at the beginning of October 2012, it is 49% above trend, down from 50% at the end of the previous month. In sharp contrast, the major troughs of the past saw declines in excess of 50% below the trend. If the current S&P 500 were sitting squarely on the regression, it would be around the 967 level. If the index should decline over the next few years to a level comparable to previous major bottoms, it would fall to the mid-400s.

Footnote on Calculating Regression: The regressions on the Excel charts above are exponential regressions to match the logarithmic vertical axis. I used the Excel Growth function to draw the lines. The percentages above and below the regression are the calculated as the real average of daily closes for the month in question divided by the Growth function value for that month minus 1. For example, the monthly average of daily closes last month was 1,437.82. The Growth function value for the month was 967.3. Thus, the former divided by the latter minus 1 equals 48.64%, which I rounded to 49%.

Footnote on the S&P Composite: For readers unfamiliar with this index, see this article for some background information.

Source: Regression To Trend: A Perspective On Long-Term Market Performance