Market's wild ride isn't over yet

The speed (or lack thereof) with which the S&P 500 finally succumbed to the 5-percent decline threshold may offer a clue to the likely magnitude of the overall decline.

The speed of decline, in our opinion, is reflective of investor behavior. A sharp and swift decline – like yanking off a Band-Aid – ends up being over quickly since investors acknowledge with a nervous chuckle that they overreacted. Hence, during the 19 times that the S&P 500 fell the first 5 percent in nine days or fewer, only 21 percent of these declines slipped into correction mode (a decline of 10 percent to 19.9 percent) and none evolved into a bear market (a decline of 20 percent+). Conversely, for those 16 times that the S&P 500 took its sweet time (40 days or more) to fall 5 percent, the "500" slid into correction mode only once, and never became a bear market. The rationale is that a drawn-out decline gives investors time to evaluate the concerns and dissipate the impact.

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It took the index 92 calendar days to fall through the 5-percent decline threshold, categorizing it – at least for now – as a pullback (a drop of 5.0 percent to 9.9 percent).

This is the longest it has taken the S&P 500 to decline 5 percent since WWII. In the prior two times, it took more than 80 days to cross the 5-percent decline threshold, one fell only 6.2 percent, while the other dropped a total of 14.8 percent. This time around, a correction is more likely, since the "500" has not experienced a decline in excess of 10 percent for more than 47 months, versus the average of 18 months since 1946.

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So there you have it: Investors have endured a white-knuckle ride on the S&P Cyclone that may not be over. History says, but does not guarantee, however, that the S&P 500 will likely not slip into a new bear market, based on the length of time the S&P 500 took to fall by 5 percent since the prior market top, combined with the strength of the U.S. housing market and today's low level of inflation.

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Even though a correction is the most likely outcome, in our view, investors are advised not to try to time the market, since corrections required an average of only four months to get back to break even. You may get out in time to avoid further carnage, but you probably won't know when to get back in, and will likely do so at a level higher than where you exited. That said, depending on the depth and duration of this decline, two stock market records are at risk. Should the S&P 500 indeed slip into a correction, depending on when we bottom, the four months typically required to get back to break even might result in a negative performance for the "500" in 2015. A down year for the S&P 500 in 2015 would be the first time since 1945 that the third year of the Presidential cycle would be negative, and the first time since 1905 that a year ending in "5" closed lower.

Commentary by Sam Stovall, U.S. equity strategist, S&P Capital IQ and and author of The Seven Rules of Wall Street. Follow him on Twitter @StovallSPCAPIQ