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 end of December, the inflation-adjusted S&P 500 index price was 187% above its long-term trend, up from November.
At the time, the peak in 2000 marked an unprecedented 109% overshooting of the trend — substantially above the overshoot in 1929 (78%) and in 1901 (93%). In recent years, the index got as high as 168% above the trend and is currently sitting at an all-time high of 187% above the trend.
From this chart, we can see that 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 28% below 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 2,097.
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.
From January 2021 to April 2022, the trend experienced 3SD above the trend but then quickly dropped back to the 2SD range previously seen at the turn of the century. However, at the start of 2024, the trend moved back into 3SD territory and closed out the year in the unprecedented 4SD range.
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.
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