Publicly funded four-year-old preschool (or 4K) programs have expanded rapidly over the past several decades, though the structure and effects of these programs vary. One key dimension by which public preschool programs vary is the length of the school day. This study investigates differences in learning between students attending full-day 4K and those attending half-day 4K in the Madison Metropolitan School District (MMSD). Following two cohorts of students (2021-22 and 2022-23), we use difference-in-differences to estimate the effect of attending full-day 4K on literacy, numeracy, and executive functioning. We also follow a subsample of students into five-year-old Kindergarten (5K) to estimate effects of full-day 4K after exiting the program. We find for the first-year cohort (N=406), there is no difference in achievement gains in literacy, numeracy, or executive function between students in full- and half-day 4K classrooms. In the second-year cohort (N=341), students in full-day 4K see larger gains in literacy, though results are not statistically significant. We do not find delayed effects of attending full-day 4K for the subsample of students in 5K (N=119). This study adds to a growing body of literature investigating variations in the length of the school day in early childhood settings.

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