Earlier quoted context omitted.
that's effectively only ~4 observations then (100/24). i wouldn't want to make a very important life decision based on data from only 4 observations. with only 4 observations, "common sense" and other forms of first-principles reasoning are much more important than the 4 data points. also one should consider data from other countries and other historical eras.
It's not though. There are ~75 24-year windows you can use between 1913 and today. I suspect you will find similar results with almost all of those windows. Obviously past performance is not an indicator of the future, but 75 data points is a pretty decent amount of data.
Some observations:
-there are 122 24-year windows in the dataset
-not a single 24-year period had a negative return
-the lowest average return for a 24-year window was 0.15% in the window from 1874-1897
-there have been only two 24-year windows since 1876 with average annualized returns less than 2%
-every window since 1912 has been at least 3%
-every window since 1968 has been at least 7.1% (the author's assumption)
-every window since 1971 has been at least 8%
Note that this doesn't take inflation into account, but also does not factor in the returns from reinvested dividends. Those effects would probably approximately cancel each other out, but hard to say.