**Quick take**: At the end of September, t*he inflation-adjusted S&P 500 index price was 170% above its long-term trend, up from August.*

About the only certainty in the stock market is that, over the long haul, overperformance turns into underperformance 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. We're 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 1.97%.

At the time, the peak in 2000 marked an unprecedented 110% overshooting of the trend — substantially above the overshoot in 1929 (78%) and in 1901 (93%). In recent years, we have seen the index get as high as 170% above the trend. The index has been above trend for nearly three decades, with one exception: from October 2008 to August 2009 the index dropped as low as 27% below trend.

At the end of September 2024, it is **170% above trend**. 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, its value would be 2080.

Incidentally, the standard deviation for prices above and below trend is about 45%. Here is a close-up of the regression values with the regression itself shown as the zero line. I've highlighted the standard deviations in red. We can see that the early 20th-century real price peaks occurred at around the second deviation. Troughs prior to 2009 have been more than a standard deviation below trend.

The trend experienced third standard deviations (3SD) above the trend from January 2021 to April 2022 but then quickly dropped back down to the 2SD range previously seen at the turn of the century. Over the past 9 months, the trend has been 3SD above the trend (current value trending close to 4SD).

ETFs associated with the S&P 500 include: iShares Core S&P 500 ETF (IVV), SPDR S&P 500 ETF Trust (SPY), Vanguard S&P 500 ETF (VOO), and SPDR Portfolio S&P 500 ETF (SPLG).

**Footnote on calculating the regression:** The regression on the Excel chart above is an exponential regression to match the logarithmic vertical axis. I used the Excel growth function to draw the line. The percentages above and below the regression are 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, if the monthly average of daily closes for a given month was 2,000. The growth function value for the month was 1,000. Thus, the former divided by the latter minus 1 equals 100%.

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

Read more updates by Jen Nash