I'm a real fan of index fund investing for the average person; it seems to me to be the only viable way for folks who don't have business-type education or experience to reliably save for retirement. However, I think it's not enough to get folks to just start pitching into a fund, they need to be motivated to stay the course. So, I got some S&P 500 performance data (Compound Annual Growth Rate (CAGR), 1871-2014, from CAGR of the Stock Market: Annualized Returns of the S&P 500) and did some what-if rates of return for various terms using sequences from the data. Let me know if this is a previously presented exercise, but it was very insightful to my thinking.
I wrote a perl script (yeah, I'm that old) that takes a particular investment duration, say 10 years, and starts with the earliest year in the data, works up an average rate of return for the subsequent period, does the same thing for the next earliest year, and proceeds until it can't fit a 10-year sequence in the remaining years. Here's the outcomes for 10, 20, 30, and 40-year sequences:
The average rate of return is simply the average of the individual S%P 500 CAGRs for each of the x years in the sequence. I was surprised to find the consistency in the rates for each of 10, 20, 30, and 40-year sequences, but I knew there had to be a discriminator. So, I added a standard deviation calculation and ta-da!, there's the difference. If you're not familiar with standard deviation, it tells you how "collected" the data you're averaging is, around the average. Specifically, if you add and subtract the standard deviation from the average, you get two numbers within which about 67% of the data resides.
So, for the 10-year average of 11.91%, 67% of the returns fall between -14.52% and 38.34%. Yes, some of the outcomes are negative. But, for the 20, 30, and 40-year runs, 67% of the outcomes are positive (well, for 20 years, only by a little). The takeaway from this is that, the longer you let your money sit in a S&P 500 fund, the greater your chances for a greater return. When I talk to young folk about this, I tend to use a 5% expected RoR, and that seems to fit in a conservative assessment that a 30-year period gives you a 67% chance of 5% or greater on your investment.
So, it's not quite enough to point folks at an index fund, they also need to be cognizant of the need to keep the investment there for a rather long time. I think telling them that you need to bake your investment for 30 or more years to have a high chance of scoring a 5% rate of return is something they can start to understand, maybe... ?
Your comments and insights are most welcome...
I wrote a perl script (yeah, I'm that old) that takes a particular investment duration, say 10 years, and starts with the earliest year in the data, works up an average rate of return for the subsequent period, does the same thing for the next earliest year, and proceeds until it can't fit a 10-year sequence in the remaining years. Here's the outcomes for 10, 20, 30, and 40-year sequences:
Years | Cases | AvgRoR | StdDev |
10 | 134 | 11.91% | 26.43% |
20 | 124 | 11.61% | 10.45% |
30 | 114 | 11.54% | 5.27% |
40 | 104 | 11.4% | 4.13% |
So, for the 10-year average of 11.91%, 67% of the returns fall between -14.52% and 38.34%. Yes, some of the outcomes are negative. But, for the 20, 30, and 40-year runs, 67% of the outcomes are positive (well, for 20 years, only by a little). The takeaway from this is that, the longer you let your money sit in a S&P 500 fund, the greater your chances for a greater return. When I talk to young folk about this, I tend to use a 5% expected RoR, and that seems to fit in a conservative assessment that a 30-year period gives you a 67% chance of 5% or greater on your investment.
So, it's not quite enough to point folks at an index fund, they also need to be cognizant of the need to keep the investment there for a rather long time. I think telling them that you need to bake your investment for 30 or more years to have a high chance of scoring a 5% rate of return is something they can start to understand, maybe... ?
Your comments and insights are most welcome...