Decoding 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. Although decoding typically becomes less constraining as reading develops, struggling readers may continue to face a decoding bottleneck into later primary years. Few randomised controlled trials have tested whether intervention-induced decoding gains translate into longer-term comprehension improvements. This study examined whether a multi-component literacy intervention can produce immediate and sustained benefits, and whether decoding gains mediate later comprehension outcomes.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 comprehension. Literacy outcomes were assessed at baseline (t1), post-intervention (t2), and at four-month follow-up (t3). The trial was pre-registered (ISRCTN18940975).Results:Children receiving RILL showed significantly greater gains in decoding (d = .19, p < .001) and reading comprehension (d = .23, p = .011) immediately post-intervention. These effects were sustained four months later, with continued advantages for decoding (d = .17, p = .004) and comprehension (d = .25, p = .010). Notably, comprehension gains were stronger at follow-up than at post-test. Mediation analyses indicated a significant indirect effect of the intervention on comprehension at t3 via decoding at t2 (y-standardised indirect β = ..19, 95% CI [.13, .25], p < .001); the direct effect was not significant (y-standardised β = .01, 95% CI [-.20, .20]).Conclusions:This is the first trial to show, in older struggling readers, that intervention-induced decoding gains mediate lasting improvements in reading comprehension. The findings highlight the persistence of decoding constraints in later primary years and show that strengthening word-level reading skills can produce enduring comprehension benefits.