## 2 Materials and Method
### Outcomes  
The outcome measure was the number of newly learned target words presented during the intervention, measured with a specially designed criterion-referenced one-out-of-four picture selection receptive vocabulary task. The task was administered before intervention started (T0), immediately following intervention (T1), and 5 weeks after intervention (T2). The task consisted of 22 target words from the theme kitchen and 22 control words. The 'kitchen' theme was chosen because it would fit the experience and vocabulary development of 3-year-old children with DLD. The control words were added to the vocabulary task because we wanted to control for incidental word learning. The control words came from the theme 'house' or 'household'. The exposure to these words outside the therapy condition (at home) was regarded as comparable to words from the kitchen theme. The control words were not presented to the children during the intervention sessions.  
The vocabulary task contained different word classes. Previous studies have suggested that out of 10 new words selected for vocabulary therapy, not only nouns, but at least 4 verbs or adverbs should be selected, because children with DLD appear to have more difficulties learning verbs than nouns [23]. Following the Dutch handbook on therapy for children with DLD [24], 60% nouns, 30% verbs, and 10% adverbs were selected as the target and control items for the vocabulary task. Several vocabulary lists were consulted to select appropriate words for our age group of children with DLD [24; 25]. Words were selected that could be presented in both intervention conditions that suited the zone of proximal development for 3-year-old children with DLD and could be depicted as recognizable images in a picture selection task. In addition, some additional words were selected that were clearly presented in the tablet game and could also be presented in the real objects condition.  
A consensus procedure within the research team, consisting of SLPs and clinical linguists, was used to select the test items. When consensus on an item could not be reached, the item was dropped, and another word was chosen. A list of the target and control words can be found in Appendix A. Because the picture selection task had to be administered three times within a short time period, there was a possible learning effect. Therefore, three different versions of the vocabulary task were constructed. In each version, the items were presented in a different order, and the four pictures on each page were also scrambled. In total, the picture selection vocabulary test contained 44 items. Photographs were used to depict the words, and all images were controlled for size, sharpness, color, and overall uniformity. Each page contained a picture of a target or control word, accompanied by three distractors, either semantically related, phonologically related, or unrelated to the target or control word. Each version of the vocabulary task contained two practice items that were presented first. During the practice phase, extra instruction could be given to the children when necessary. The children were instructed to point at the picture that matched the word best.  
The researchers that administered the picture selection tasks were blind to the intervention condition the children were assigned to during all three measurements (T0, T1, and T2). In general, the same researcher assessed the same group of children. In a few cases, when children were absent during assessment, an SLP from that special daycare center who was not involved in the study administered the task.