Long-Term Stock Market Investing: Chance Of Success?

If you invest in the S&P 500 index, what's the chance it'll pay off if you wait 20 years? With this New York Times chart, you can see how the S&P 500 index has gained and lost for any time period since 1920. As can be seen, it depends a lot on the starting and ending times. Also, it should be noted that one can't assume the future will match this history. Hat tip to My Money Blog for mentioning the chart.