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.

The market giveth and the market taketh away. US stocks returned roughly 3,000% in total from 1979 through 1999 — then gave back 28% of their value, in total, from 2000 through 2008.

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.