The excess return of common stocks over short Treasuries is a controversial number these days. The number in the table is a traditional value based on historical returns in the US market, but recent research suggests the traditional values are much too high. One piece of research looks at historical returns across countries and notes that the US has been extremely lucky. Another line of research challenging the traditional numbers looks at forward-looking estimates based on analysts' forecasts.

The reason there is still controversy about the mean return on equities, in spite of many years' experience, is the high level of volatility in stock markets. This is also why performance measurement is difficult, if not futile.