Better than a Crystal Ball: See How Your Investments Will Grow
July 16th, 2008
When I took Dave Ramsey’s Financial Peace Class he had one lesson on investments. He was careful to explain that he was not an investment expert. Most of his advice concerned saving regularly and putting money away in IRAs and 401(k)s. But several times he said that his investments in “good quality growth mutual funds” were returning 15% annually, and you, too, should expect that. Probably at the time he wrote that lesson, which was during a very good stock market run, he was getting those returns.
Today, he might admit to being a victim of what’s called recency bias. That means you look into your crystal ball and see recent events continuing into the future. The most recent events carry more weight than those in the more distant past.
This happened to me in one of my first jobs. I had a very good month; a difficult project finally came together, and my numbers were really high. My annual performance review was the next week and I was expecting a great review. But my manager didn’t just focus on the last month. The 11 months before that weren’t so good, and the one good month probably kept me from being fired. Recency bias caused me to be overconfident. The next year I learned about another force called reversion to the mean. An extreme event is temporary and the next event is likely to be closer to the average performance. My extremely good month was followed by several more mediocre ones. I didn’t last much longer in that job.
Reversion to the mean also explains why Manu Ginobili can have a very poor performance in one game and his teammates are confident he will have a very good game the next day. If his average is 19 points per game, and he only scores 6 one night, he likely will revert to the mean and score 19 the next night.
What does this have to do with your investments? It means that you can use the long-term averages for different types of investments to forecast the return of your particular portfolio. If you have a diversified portfolio of U.S. stocks, international stocks, and bonds, you can estimate the return for each category. You won’t be susceptible to recency bias and assume that a particular investment will always be going up (or going down.) You won’t sell your bonds and buy foreign stocks if the internationals have an up year. And you’ll realize that if one part of your portfolio has done extremely poorly, it will probably revert to the mean and perform as expected the next few years. And after what we’ve experienced so far this year, any reversion to the mean will be welcome.




