Friday, April 18, 2008

Bandwagon Effects in the Stock Market

A little while back I posted about the increasing number of one-sided days in the stock market and how those are affecting traders. Yesterday I noticed another kind of one-sidedness: seven of the last nine trading sessions in the S&P 500 Index ($SPX) have closed either in the top or bottom quarter of their day's range.

This led me to examine more broadly the issue of how often stock indexes close near their day's highs or lows. If we divide each day's range into quartiles, then we would assume, over time, that a chance distribution would put 50% of market closes in the top or bottom quarter of the day's range and 50% in the middle two quarters.

That is not what we see in the data, however. Since July, 2007, two-thirds of all trading days closed either in the day's top or bottom quarter: 132 out of 200 days, to be precise. From January, 2000 through June, 2007, that ratio was 61% (385 out of 627 days).

If we tighten the criteria and divide each day's range into deciles, we can observe how often the market closes in the top or bottom 10% of its daily range. One would expect, by chance, that this would occur 20% of the time. Since July, 2007, however, it has occurred 77 out of 200 days: 38% of all occasions. From 2000 through June, 2006, we closed in either the top or bottom 10% of the daily range in $SPX 209 out of 627 days, or 33% of the time.

What we are seeing is that markets are closing near their day's highs or lows more frequently than we would expect by chance. This may reflect a bandwagon effect, in which traders and investors observe market movements during the day and don't want to miss out on them. This would lead them to buy rising markets and sell falling ones, creating late day strength or weakness. Regret and the fear of missing out on a market move would lead to increased trending behavior as the day progresses, creating a kind of one-sidedness to the trade.

This bandwagon effect, exaggerating market movements late in the day, tends to be unwound the next day. Going back to 2000 (N = 2082 trading days), we find the following average next day changes in $SPX as a function of the location of the prior day's close:

Prior Day Closes in Top Quarter of Range (N = 727): -.06% (343 up, 384 down)
Prior Day Closes in Middle Two Quarters of Range (N = 743): -.02% (381 up, 362 down)
Prior Day Closes in Bottom Quarter of Range (N = 612): .11% (357 up, 255 down)

What this suggests is that, once a bandwagon starts during the day, it tends to persist into the close. Fading one-sided days, particularly of late, has not been a fruitful endeavor for traders. Expecting bandwagons to persist into the next day's trade, however, has also not been profitable. It appears that traders segment their performance day by day, perhaps jumping aboard trends in an effort to finish their days on winning notes. By the close of the next day, however, any such bandwagon effect has been erased.


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