It's something you'll hear in your entry-level courses in finance or investing: Stocks on average return about 10% a year, and bonds return about 5%.

Unfortunately, in any given year, average almost never happens. Just get it out of your head.

Those averages belie the incredible amount of volatility that has historically occurred and will most likely occur again.

Vanguard recently tweeted this scatterplot charting annual stock and bond returns since 1926. The gold bar covers average stock market returns and the silver bar covers average bond market returns.

(@Vanguard_FA)

"Since 1926, the average annual return for US stocks has been a little more than 10%, and this seems to be pretty common knowledge for even neophyte investors," Vanguard analyst Donald Bennyhoff wrote. "So 10% would seem to be a reasonable expectation for an average year, right? Our illustration ... shows how often that assumption is erroneous, but it is also irrelevant."

Again, just get it out of your head that a year may turn out to be an average year.

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