The reciprocal importance of vocabulary development for reading comprehension and educational achievement is well established (Biemiller & Boote, 2006; Beck& McKeown, 2007; Cain, 2010). Early vocabulary has been found to predict emergent word decoding (Wolff &Gustafsson, 2016) and reading comprehension  and decoding ability after grade two to predict later vocabulary development (Verhoeven, van Leeuwee & Vermeer, 2011). Productive vocabulary at age 8 has also been found to account for variance in word reading four years later (Nation & Snowling, 2004). Individual differences in vocabulary have also been repeatedly found to correlate with factors of socio-economic status and home educational resources (Hart & Risley, 1995; 2003 Rindermann & Baumeister, 2015).  

While the close relationship between vocabulary and reading comprehension has been known and repeatedly corroborated over the past century (Stahl & Fairbanks, 1986) explicit instruction of word meanings beyond occasional subject related words, has been found to be sporadic (Lesaux, Kieffer, Faller & Kelley, 2011).

However, in recent decades, intervention studies have attempted to ascertain efficient methods to increase vocabulary knowledge among students (Graves & Silverman, 2011). Results are inconclusive but there is growing consensus that the most effective methods of vocabulary instruction are those using multifaceted strategies to further vocabulary breadth and depth (Christ & Wang 2011; Stahl & Nagy, 2006; Webb, 2009).  

In a review of interventions targeting receptive vocabulary in early ages, Marulis and Neuman (2010) found an overall effect size of .88. The review covered 67 experimental or quasi-experimental interventions targeting children without neuropsychiatric or developmental diagnoses (N=5929). The age of the participants ranged from birth to age 9. At risk children, for example children with dyslexia, from middle- and upper income level families were found to benefit the most from the intervention (Marulis & Neuman, 2010). Another intervention study, targeting middle school students (N=476) in grade 6 with a high percentage of language minority learners (Lesaux, Kieffer, Faller & Kelley, 2010) reported lower but significant effect sizes for word meaning (.39) as well as depth of word knowledge (.15). The latter study also found significant (.15) transfer effects on reading comprehension (Lesaux, Kieffer, Faller & Kelley, 2010).

Transfer effects of word knowledge from trained to un-trained words has been linked to specific methods of instruction. In a recent study involving 132 Norwegian students in grades 1 to 8, a structured and detailed training program was found to facilitate transfer effects on untrained words while a less structured training program did not (Frost, Ottem, Hagtvet & Snow, 2015). The two compared methods yielded no significant difference for the trained words (Frost et al., 2015).

The objective of this study is to develop and evaluate a structured vocabulary intervention program designed to increase vocabulary size and facilitate reading comprehension in a Swedish context. The study design is a cluster randomized trial comprising 600 second grade students in Sweden. This first part of the study includes data from 225 students. The intervention is evaluated using tests on vocabulary, reading comprehension, spelling and decoding as well as non-verbal cognitive abilities. Questionnaires to students, teachers and parents provide referential information for the analysis.

The research questions are: 1) what are the preliminary effects of a structured vocabulary intervention in grade 2 on trained and untrained words? 2) What are the preliminary effects of a structured vocabulary intervention on reading comprehension? 3) are there mediating effects of home background or gender on vocabulary and reading comprehension?

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