Event study estimates of the effect of hydroxyurea use on annual hemoglobin values. Data for the entire sample (top panel) and for the treatment group limited to those with laboratory evidence (ie, increase in MCV) of hydroxyurea adherence (bottom panel) are shown. Points with 95% CIs (y-axis) show the estimated average treatment effect for those treated with hydroxyurea for the given year of therapy (x-axis) relative to those who never used hydroxyurea. Year 0 is the year of hydroxyurea initiation.