Delayed high school start times later than 8:30am and impact on graduation rates and attendance rates

29 high schools across 8 districts in 7 states, total membership over 30,000 students

US public high school students in districts that moved start times to later than 8:30 a.m.

Pre-post repeated-measures ANOVA on school-level graduation and attendance rates two years after a delayed start was implemented. There is NO comparison group of unchanged schools — the within-school pre-post design is the entire identification. Graduation rates were rising nationally over the study window, so a secular trend alone would produce this result. The paper is nonetheless one of the most cited pieces of evidence in start-time advocacy.

Attendance rates and graduation rates significantly improved in schools with start times of 8:30 a.m. or later. The authors go on to raise the question of whether later start times are "a mechanism for closing the achievement gap due to improved graduation rates" — a claim that a pre-post design with no control group cannot support.

Not the binding problem here; the binding problem is the absence of any counterfactual. But note that the 29 schools were districts that CHOSE to move their bell times, a self-selected set of well-resourced districts likely also making other simultaneous improvements.