Frequent engagement in extramural English (EE) activities (i.e., out-of-school English-language activities) has been shown to positively influence high school students’ vocabulary size (Sundqvist 2009). However, given the focus on receptive knowledge in previous studies, our understanding of the relationship between EE activities and students’ production remains somewhat rudimentary. What is more, syntactic and broader lexical aspects have received very limited focus. Against this background, the present study examines the effect of EE activities on both lexical diversity and NP complexity in high school student writing. The following research questions are investigated:   

• What effect (if any) does EE activities have on lexical diversity and/or NP complexity?
• Are there differences between purely receptive EE activities and other types of EE activities in terms of the effect of lexical diversity and NP complexity, and, if so, what are the differences? 

A subsample of the Swedish Learner English Corpus (SLEC) is used (grades 10–11; n=200). SLEC contains detailed information about how many hours per week students engage in five different EE activities (reading, watching, conversation, social media, gaming). 

To measure lexical diversity, moving average type-token ratio is used. To measure NP complexity, the rate of occurrence of attributive adjectives and prepositional phrases as modifiers in NPs is used. 

In order to test the effect of EE on lexical diversity and NP complexity, we applied measured variable path analysis from the Structural Equation Modeling framework. The best-fitting model (χ2: 16.8, df: 15, CFI: 96.6, RMSEA: 0.023[0.00–0.067], SRMR: 0.039) confirmed our hypotheses that (a) participation in EE activities has a (mostly) positive effect on lexical diversity and NP complexity, (b) that the activities grouped differently based on type, where the purely receptive activities (in particular reading) each had an effect on lexical diversity, in a way that the other EE activities did not.