Decoding and vocabulary improvements mediate sustained gains in reading comprehension: Evidence from a randomised controlled trial of a multi-component reading intervention

Background:
Reading comprehension is critical for academic success, yet many children with persistent decoding difficulties struggle to achieve it. This study examined whether a multi-component literacy intervention is effective in improving reading comprehension, and whether any gains in comprehension are mediated by improvements in word reading and vocabulary knowledge.
Methods:
In a randomised controlled trial, 285 English-speaking children aged 7–9 years with reading difficulties were assigned to a waitlist control group or the Research Informed Literacy with Language (RILL) intervention, a structured, multi-component programme targeting decoding and language skills. Literacy outcomes were assessed at baseline (t1), post-intervention (t2), and at four-month follow-up (t3). The trial was pre-registered; https://www.isrctn.com/ISRCTN18940975.
Results:
Children receiving RILL showed significantly greater gains in word-level literacy (d = .19, p < .001), taught vocabulary (d = .30, p = .017) and reading comprehension (d = .23, p = .011) immediately post-intervention. Effects were sustained at follow-up (word-level literacy d = .17; taught vocabulary d = .30; comprehension d = .25). Mediation analyses, showed a significant indirect effect of the intervention on comprehension at delayed follow up via word-level literacy at t2 (y-standardised indirect β = .10, 95% CI [.06, .12]), with a negligible direct effect (y-standardised β = .01, 95% CI [-.20, .20]). In an additional exploratory parallel-mediation model, both t2 word-level literacy and taught vocabulary showed unique indirect effects on t3 comprehension (word-level literacy indirect: β = .16, 95% CI .10, .21, and taught vocabulary indirect β = .11, 95% CI .02, .23).
Conclusion:
Our intervention produced immediate and sustained improvements in word-level literacy, taught vocabulary, and reading comprehension in struggling readers. Persisting decoding weaknesses are common in later primary years and our findings show that improving word reading can produce enduring benefits for comprehension.