Is it true that, "just ten trading days represent 63 per cent of the returns of the past 50 years"?

External question — Personal Finance Stack Exchange Author: KDecker Original post: https://money.stackexchange.com/questions/114772 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. I have fallen down a Wikipedia rabbit hole and landed on the page titled Seven States of Randomness . I can't explain in a single sentence what it is talking about, but my question is about an odd quote at the end of the History section (with my emphasis) Mandelbrot and Taleb pointed out that although one can assume that the odds of finding a person who is several miles tall are extremely low, similar excessive observations can not be excluded in other areas of application. They argued that while traditional bell curves may provide a satisfactory representation of height and weight in the population, they do not provide a suitable modeling mechanism for market risks or returns, where just ten trading days represent 63 per cent of the returns of the past 50 years. Is this true? Or is it even fair to ask if this is true? Does anyone know where this quote originated from or is this just the made up "fact" of whoever wrote this Wikipedia page? If it is true, is there a better less technical explanation of it somewhere?