Outliers can distort the yearly average, which makes the 10% average an unreliable predictor of stock market performance for a given year. Outlier years are those in which the return is significantly greater or lower than typical. For instance, compared to the return for the previous ten years, which was 12.39%, the average stock market return for the past twenty years, 9.75%, may appear a little low. And it’s not as if the years 2002 to mid-2023 were particularly horrible. The average return was 26.38% in 2003 and 23.45% in 2009. The 20-year average, however, was impacted by negative outliers.