Event study estimates of the effect of hydroxyurea use on annual ED visits. Points with 95% CIs (y-axis) show the estimated average treatment effect on those treated with hydroxyurea for the given year of therapy (x-axis) relative to not using hydroxyurea. Year 0 is the year of hydroxyurea initiation. Of note, the DiD estimate for this analysis was −0.36 (95% CI, −0.57 to −0.16), meaning, on average (across treatment year and age group), that hydroxyurea use was associated with 0.36 fewer ED visit per year.