Question 3: intervention properties
Concerning the third research question of intervention properties, we asked whether in-game metrics are relevant predictors for response to GraphoGame-NL intervention. Studies reporting positive GraphoGame-related effects used training durations ranging from one up to 28 weeks with an intensity of two to five training sessions per week (Previous literacy digital game-based learning studies using GraphoGame usually rely exclusively on the number of gaming sessions, or the time spent playing as a measure of training intensity. Only a few studies communicate treatment fidelity measures such as attrition rates, which can be as high as 46% (Possibly, the rather strict inclusion criterion of at least 20 playing sessions made the present sample too homogenous to find interactions between intervention outcomes and exposure. Upon re-inclusion of children who played less than 20 sessions and by combining exposure measures with a principal component analysis, we found that game exposure modulated reading fluency when phonological awareness and rapid automatised naming were statistically controlled. For the maturation of literacy skills, we argue that the time-course of development of phonological skills plays a crucial role for the benefits of GraphoGame‑NL. Playing beyond mastery of grapheme-phoneme correspondences has little impact on reading fluency when phonological skills are poor. Children with good phonological skills at pre-test benefited more from the exposure to GraphoGame-NL than children with poor phonological skills. We suggest reducing the weekly playing intensity once letter-sound knowledge accuracy reaches ceiling, and extending the overall training period. This might allow poor performers to get more out of the game, especially to give more time for maturation of phonological skills (see 