This study evaluates a "nudge letter" to parents intervention designed to reduce chronic absenteeism among students in one urban district. Using a regression discontinuity design (RDD), it estimates the impact of the intervention on improving student attendance. The forcing variable for the RDD was 2016-2017 attendance rate, with a "threshold" of a 0.90 attendance rate (missing 10% of days). Analyses established demographic equivalence of students in the 0.88 to 0.92 baseline attendance bandwidth. Although the overall impact of the intervention on attendance change between Fall 2016 and Fall 2017 (first-quarter attendance) was small and non-significant (ES 0.09, p = 0.20), the effect size for middle school students (0.34, p = 0.044) was "substantively important" by What Works Clearinghouse standards. The effect of the intervention on the full year's attendance rate was not significant. [For the corresponding grantee submission, see ED608191.]