Investors looking for good news today can take solace knowing that Friday the 13th is not especially unlucky.

Indeed, researchers in the Netherlands have determined that fewer accidents and reports of fire and theft occur on Friday the 13th than other Fridays. And stocks are actually trading up in early market action. Still, some people won't care. About $800 million to $900 million will be lost in business today because people will not do things they normally do.

Investors need to remember that there are many ludicrous theories about the stock market. There is the Super Bowl Indicator where people figure that if a team from the old American Football Conference (now the American Football Conference) wins, the market is headed down, while a win for the old NFL (now the National Football Conference) means good times are ahead. People believe that bad things happen in October and that May is the time to sell and go away.

Sometimes these theories "work." Other times they don't. None of them should serve as the sole basis for any investing decision. I understand their appeal because they seem to take the guesswork out of figuring out the gyrations of the market. Real life, though, does not always fit into theories.

The Dow Jones industrial average and the NASDAQ Composite Index are both down more than 8% this year. Gasoline prices have topped $4, sending many trucking companies to the brink of bankruptcy. Soaring commodity prices have squeezed profits of businesses ranging from Dow Chemical Co. (NYSE: DOW) to the local pizzeria.

If you want to invest, do your homework. Of course, people still need to avoid black cats crossing their path, stepping on sidewalk cracks and breaking mirrors.