I was trying to read the triumph of the optimist, but it was almost impossible to see a well-written formula to show how the returns have been computed. In a simple sense, I do not know how the annual return index have been computed? I am also not sure whether natural logarithm has been applied to say (P(t)-P(t-1))/ln(p(t-1)). If anyone has encountered this argument please refer me to a page or help us understand whether the returns found in Ibbotson associate is natural logarithm or simply simple returns?
These data are commonly used for long-term investors such as in pension funds, They also seem to have been called low-frequency returns.