The worst September since 2008 How that affects October

September is usually the worst month for stocks, but this September looks like it will be the worst since 2008, when the financial crisis was at its worst. Investors who pay attention to seasons might wonder what this means for October.

Dow Jones Market Data looked at how well stocks did after particularly bad Septembers in the past.

But first, how does this month compare to the rest? As of the end of trading on Thursday, the S&P 500 SPX, which was down 7.95%, and the Nasdaq Composite COMP, which was down 9.1%, were on track to have their worst Septembers since 2008. With a 0.07 percent drop and a 7.25 percent drop, the Dow Jones Industrial Average DJIA was on track for its worst September since 2002.

Sample sizes are small. If you don't count this September, data going back to 1928 shows that the S&P 500 fell by 7% or more 11 times before. Since 1928, the Dow has dropped by 7% or more 13 times in September. Since 1986, the Nasdaq Composite has dropped by 9% or more six times in September.

According to Dow Jones Market Data, when the S& P 500 fell by 7% or more in September, it rose by 0.53% on average and 1.81% on average in October. That's better than the average of 0.47 percent for all Octobers and the median of 1.03 percent. October is good 54.55% of the time after a big loss in September, but only 57.45% of the time for all Octobers.

Dow Jones Market Data found that after a drop of 7% or more in September, the Dow fell by 1.51% on average in October and 1.46% on average in November. The average gain for all Octobers is 0.37 percent, and the median gain is 0.79 percent. Following a 7% or greater drop in September, the S&P 500 has risen 46.15 percent of the time in October, compared to 57.6 percent of the time in all Octobers.

The Stock Trader's Almanac says that since 1950, September has been the worst month for the Dow Jones Industrial Average, S&P 500, and Russell 1000. It has also been the worst month for the Nasdaq Composite since 1971 and the small-cap Russell 2000 since 1979.

On Friday, stocks were mostly up, but trade was choppy. The Dow was up 21 points, or 0.1%, while the S&P 500 was up 0.4% and the Nasdaq was up 0.8%.