As mentioned previously, the present study is based on empirical data collected in the context of a broad longitudinal investigation concomitant to the Berlin bilingual education programme. It covers signed and written narratives elicited on the basis of the picture story book “Frog, where are you?” (cf. Mayer 1969).
Picture based narrative elicitation. The picture book elicitation procedure was chosen despite the widespread practice of collecting spontaneous data in longitudinal investigations of (spoken) language acquisition. It was deemed of advantage to have a certain control on the events referred to by the narrator, not only for the comparison of participants’ productions across time, but also for the assessment of their competence in the two languages they were acquiring. As for the choice of a picture book vis-a-vis film viewing, we considered the former more appropriate, in particular for the first years of the investigation, covered in the present study, because of the additional memory burden the latter might have involved (cf. Berman & Slobin 1994: 40-41).11 
At the same time, we acknowledge that what sets this type of elicitation procedure apart is that the narrative genre produced is characterised by “verbalization of graphic representation of non-veridical, fictive sequences of events” (Berman 2004: 262). It is interesting to note that in literate Western cultures, even young children aged 3-4 have been found to be able to “translate static, graphic material into dynamic verbal event descriptions”, and children aged 5-6 to produce sequentially organised narratives (Berman 20 04: 263). However, only older school-age children and adults manage to produce a globally organised narrative text. These findings indicate that while children are familiarised with the task they have to accomplish their eventual narrative products depend on their command of the language and constraints on its use.
On a more general level, because elicitation procedure and situational context might affect the choice of linguistic forms used, we acknowledge that these two factors need to be considered in the interpretation of the data. We note also that caution is required in the extrapolation from a limited set of data collected under specific circumstances.
Selection of the “frog story”. The so-called frog story is a picture story that differs from short picture sets commonly used in the literature not only in length but also in its episodic complexity. Indeed, unlike short picture series confined to one main episode, the main theme of the frog story (the search of a runaway frog) is made up by many sub-episodes (Berman 2004: 268).
For the purpose of our investigation, we favoured the choice of this picture book over a small set of pictures, because, as Berman and Slobin (1994: 41) point out, a picture story with a more complex structure, as is the case of the frog story, requires to recall the progression and outcome of the story while describing the individual pictures. At the same time, because participants were not instructed to choose a particular genre, we acknowledged that participants could vary in the text type they would produce (between picture-description and story-telling or a mixture of both) (cf. Berman & Slobin 1994: 42).
Another factor that tilted the choice toward this particular story was the available literature on a broad cross-linguistic investigation into narrative development based on this picture book (cf. the contributions in Berman & Slobin 1994; and Stromqvist & Verhoeven 2004; among others). Though different in orientation (our research focuses on development and knowledge of grammar), our study has profited from the insights obtained in “frog studies” in that they helped us discern those phenomena in learner productions that needed to be regarded from the more global perspective of narrative development.
-  In the final 2 years of the longitudinal study not covered in this work we decided to changethe elicitation technique and use short clips, following the observation that students of an elderage tend to be bored by picture stories.
-  Berman and Slobin (1994: 3) remark that this story was used first by Bamberg in his dissertation because of the diversity of temporal relations that might be expressed in the retelling (theymention: sequence, simultaneity, prospection, retrospection, ongoing and competed events) (forfurther discussion see Berman & Slobin 1994: 20f.)