Correlational studies are useful for constructing hypotheses about the effects of shared reading on oral language development. In correlational research, controlling for third variables that could account for systematic differences in the outcome variables safeguards against misinterpretations. There are, however, severe limitations to the conclusions that can be drawn from correlational data because such data typically preclude causal interpretation. By contrast, intervention studies that use randomized allocations to treatment conditions and well-defined intervention programs ensure that systematic differences between intervention and control groups are most likely caused by the intended manipulations. Comparisons between intervention groups, active control groups, and passive control groups allow a differentiation of specific and non-specific intervention effects (e.g., Hawthorne effects).
Systematic reviews from What Works Clearinghouse have investigated whether children’s language development is affected by high-quality shared reading interventions in general (U.S. Department of Education, 2015) and dialogic reading in particular (U.S. Department of Education, 2007). For shared reading in general, the authors conclude that there are mixed results concerning the impact on oral
language development. Specifically, it is unclear whether shared reading interventions affect a range of oral language skills or only some of them (e.g., vocabulary; U.S. Department of Education, 2015). For dialogic reading, the authors conclude that this particular shared reading method has positive effects on oral language skills (U.S. Department of Education, 2007); however, the studies that were reviewed only investigated effects on vocabulary and grammar.
Regarding the HLE, a meta-analysis investigated the magnitude of dialogic reading intervention effects on vocabulary skills (Mol et al., 2008; see Table 5.1). All studies used randomization and non-treatment control groups. Parents participated in programs teaching and encouraging them to use dialogic reading strategies while reading with their children. Interventions lasted for 9 weeks on average (range: 4– 28). Dialogic reading in the HLE had a moderate effect, explaining 4–8% of variance in vocabulary development. Moderator analyses provided evidence that the effect was larger for preschoolers (k = 10, d = 0.50) than for kindergarteners (k = 6, d = 0.14, not significant). Moreover, children from families with a low SES did not benefit from the intervention (k = 7, d = 0.13), whereas children’s oral language skills from families with a middle SES increased significantly (k = 9, d = 0.53). As a limitation, the authors note that none of the studies controlled for the amount of shared reading activities in control groups, which is also likely to influence oral language development.
In regard to the CCLE, a meta-analysis investigated the impact of interactive shared reading interventions on preschoolers’ and kindergarteners’ oral language skills (Mol et al., 2009). All studies used randomization and control groups and implemented an intervention with interactive shared reading strategies to encourage children’s active engagement, such as open-ended questions, prompts, comments, and positive reinforcement. On average, intervention programs comprised 42 shared reading sessions (range: 4–66 sessions). The average intervention effect was moderate, explaining about 6% of variance in vocabulary development. Moderator analyses indicated that the effect was larger when researchers conducted the intervention (k = 15, d = 0.79) in comparison to teachers conducting the intervention (k = 16, d = 0.35). Further moderator analyses were conducted, but due to the limited number of studies and several problems with confounds (e.g., between intervention dosage, group size, and intervener), their explanatory power is limited. A main limitation of this meta-analysis is again that
intervention studies did not control for the amount of shared reading in the non- treatment groups (Mol et al., 2009). Furthermore, studies often did not report any measures of implementation and treatment fidelity, which is especially important when evaluating the effectiveness of interventions that were conducted by teachers. In particular, most of the studies reported the intended treatment dosage, but none reported the effective treatment dosage (Mol et al., 2009).
Updating the evidence reported by previous meta-analyses, a recent meta- analysis investigated the combined effects of shared reading interventions in the HLE and CCLE on children’s vocabulary skills (Noble et al., 2019). Interventions lasted mostly for 6 to 8 weeks (range: one week to seven months). The average intervention effect was small at posttest and non-significant at follow-up (see Table 5.1). A comparison between treatment group and passive control group yielded a larger posttest effect (k = 43; g = 0.26) than the comparison between treatment group and active control group (k = 12; g = 0.03, not significant). Other potential moderators, such as SES of participants, intervention duration, age of participants, and intervening person, were not related to intervention effects.
Of the 53 studies included in the meta-analysis, only 12 compared vocabulary development between a shared reading intervention group and an active control group (Noble et al., 2019). The authors conclude that more studies with an active control group participating in a non-language intervention are needed for an evaluation of the specificity and the magnitude of shared reading intervention effects. The medium effect sizes reported by previous meta-analyses might not represent specific shared reading intervention effects but could largely reflect non- specific intervention effects that are not attributable to shared reading activities. Moreover, studies should use an adequate intervention dosage of at least six months, include a follow-up assessment for the evaluation of sustained effects, and target a range of language outcomes to evaluate whether shared reading affects the development of different language skills (Noble et al., 2019).
In Germany, few intervention studies have investigated effects of shared reading on children’s oral language skills, and most of them targeted children with below-average German language skills or multilingual children (e.g., Buschmann & Sachse; 2018; Ennemoser & Hartung, 2017; Ennemoser, Kuhl, & Pepouna, 2013; Hartung, 2015; see Egert, Galuschka, Groth, Hasselhorn, & Sachse, 2020, for a review). Three dialogic reading studies used randomization, at least one control
group, and research assistants as interveners. Dialogic reading was effective in fostering four- to six-year-olds’ vocabulary and grammar skills (Ennemoser & Hartung, 2017; Ennemoser et al., 2013; Hartung, 2015). In addition, several intervention studies that implemented an interaction training for child care staff that uses similar communication strategies as dialogic reading reported some positive effects on children’s oral language skills (Buschmann & Sachse, 2018). In sum, there is some preliminary evidence that dialogic reading is effective in fostering children’s oral language skills in Germany (Egert et al., 2020).