Heterogeneity in treatment effects of the Head Start, a federally funded early childhood development program in the United States, has previously been found in the Head Start Impact Study (HSIS), a nationally representative randomized controlled trial. While individual characteristics have been extensively examined as sources of effect heterogeneity, treatment effects may vary as a function of outcome distribution (i.e., distributional effect). Using quantile regressions, we investigated distributional effects of the Head Start on eight child developmental outcomes for first year, second year, third year, and the 3rd grade year follow-up in the HSIS data. For PPVT and Applied Problems, the effects varied substantially across quantiles in the first follow-up, but they were positive overall. The effects at the lower quantiles were larger and were sustained beyond the first follow-up (PPVT [95% CI] at 10th and 90th quantiles: 8.74 [6.22, 11.27], 3.32 [0.82, 5.81]) in the first follow-up and 5.72 [2.66, 8.77], − 1.66 [-3.69, 0.37] in the second follow-up). For Behavior Problems, the effects were only positive for the lower quantiles in the first follow-up, but they became null in the latter follow-ups. For Letter-Word Identification, Spelling, and Pre-Academic, the effects were positive in the first follow-up with moderate variation across quantiles. In the second follow-up, only the effects at the lower quantiles were statistically significant, although they faded in the latter follow-ups. For Oral Comprehension and Social Skills, effects were null for all follow-ups. The Head Start had meaningful distributional effects for a range of child developmental outcomes, and distributional effects should be routinely assessed for better understanding of child developmental programs.
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