Historical facts

if you had invested $ 100 in the S & P 500 in 1926, you would have had $ 307,700 in 2006 — a pretty staggering gain. But if you had been out of the market for the best - performing 40 months of that lengthy 972 - month period, you would have had just $ 1,823 in 2006. That means that 99 percent of the gains over that 81 - year period came in just 4 percent of the months. The principle holds over shorter periods, as well. If you invested $ 100 in 1987, you ’ d have had $ 931 by the end of 2006, Sahadi noted. But if you were out of the market for the 17 best trading months of that 240 - month period, you ’ d have ended up with just $ 232. In this case, 84 percent of the gains came in 7 percent of the months. The bottom line: While the market rises substantially over time, much of its increases come on a relatively small portion of trading days.