Over the long run the S&P 500 has returned roughly 10% a year on average before inflation — but almost no individual year actually lands near 10%. The average hides wild swings.

Since 1928 the S&P 500 has finished the year higher about 73% of the time — roughly three years in four end in the green. Down years are impossible to predict in advance, but the base rate quietly favors staying invested.

The worst rolling 10-year total return the US stock market has ever produced is a loss of about 40%, for the decade starting at the September 1929 peak. The best rolling 10-year return is a gain of about 597% — roughly 21% a year — for the decade starting in the summer of 1949. Same market, ten-year holding period, wildly different outcomes depending only on when you started.

Miss the market’s 10 best days over a couple of decades and your long-run return can be cut roughly in half — and those best days often cluster right next to the worst ones.