US S&P 500

Recently I downloaded the 58-yr data of US S&P 500 from Yahoo Finance and did some numbers crunching out of curiosity.

I wanted to find out if there is a relation between holding for a certain specific of time, say 10 years, 20 years, 30 years.
This is what I get:-

click on image to enlarge

How to read this chart: for example, see right hand side of graph, you see Jan-08, read off the chart for the 3 colored lines on the vertical axis to get the respective specific periodical annualized returns in %.
Say, see the blue line (10-yr): this means investing in the US S&P 500 portfolio for period Jan 1998- Jan 2008 gets about 2.2% per annum. The yellow line would read about 9.2% per annum for the period Jan1978-Jan2008.

It seems to suggest that holding long term (30 years) beats the shorter 10 & 20 years. The 20-yr period just lost by a small percentage though. But there are certain periods where returns are above 15% per annum, and that was during the dotcom period 1998-1999, but this means investors would have begun investing during 1988-1989.

If "what goes up must come down and what goes down must come up" is true, then it may be a good time now to invest in US S&P 500 and wait for 10 years. However, I do suggest dripping in small money consistently and rebalancing along the way, see "Drip in new money"

Lastly, the annualized returns for the last 58-yr period is 7.8%