To the Editor: In our article “Moderate-to-Vigorous Physical Activity From Ages 9 to 15 Years,”1 an error occurred in the programming syntax used to clean the accelerometer-determined physical activity data for age 12 years and age 15 years. A subsample of days that should have been deleted as invalid (implausible total count relative to counts on valid days) was retained for some individuals. For the analysis sample at age 12 years, this had the effect of altering the moderate-to-vigorous physical activity (MVPA) estimates of 164 individuals and excluding 3 individuals. For the analysis sample at age 15 years, this had the effect of altering the MVPA estimates of 90 individuals and excluding 2 individuals. One additional individual at age 12 years was eliminated from this analysis because the revised score was identified as an extreme outlier (weekday MVPA >450 min/d). The estimated levels of MVPA at age 12 years and age 15 years increased slightly, but the statistically significant findings of decreases in daily MVPA for boys and girls between ages 9 and 15 years were not altered. Corrected text, tables, and figures are included in the Correction section of this issue of JAMA.