[dsjoerg]

Those 75 data points are not independent observations, because the windows overlap so much. Remember that using "year" as the granularity is arbitrary anyway; if we had daily, hourly or by-the-minute data going back to 1913, then we could have more "data points" but we would not have any better insight or statistical significance into the original question.

My point is that having 100 years of investment data is much more like having 4 data points than it is like having 75 or 75 *12 (if you cut the years into months). Even though you literally have 75 data points, they are pretty close to a copy-paste of each other; not statistically independent.