## 3.2 Bridging instruments by mapping items

Many instruments for measuring child development have appeared since the works of and . It is no surprise that their contents show substantial overlap. All tools assess events like starting to see, hear, smile, fetch, crawl, walk, speak, and think. We will exploit this overlap to bridge different instruments. For example, Table 3.2 displays the labels of milestones from six instruments. All items probe the ability of the child to formulate “sentences” of two words.

 Item Label by1mdd136 sentence of 2 words by2mdd114 Uses a two-word utterance ddicmm041 Says sentences with 2 words denlgd019 Combine Words grihsd217 Uses word combinations vinxxc016 use a short sentence

The idea is to check whether these milestones measure development in the same way. If this is found to be true, then we may formally restrict the difficulty levels of these milestones to be identical. This restriction provides a formal bridge between the instruments. We repeat the process for all groups of similar-looking items.

A first step in the bridging process is to group items from different instruments by similarity. As the by3 is relatively long and is the most often used instrument, it provides a convenient starting point. Subject matter experts experienced in child development mapped items from other tools to by3 items. These experts evaluated the similarity of wordings and descriptions in reference manuals. Also, they mapped same-skill items across other instruments into groups if these did not map onto by3 items.

Figure 3.1: Connections between the instruments via mapped item groups by domain (https://tnochildhealthstatistics.shinyapps.io/GCDG_mapping/).

Figure 3.1 connects similar items and hence visualises connections between instruments. Items are displayed in the wheel, coloured by instrument. We organised item mappings into five domains: fine motor (FM), gross motor (GM), cognitive (COG), receptive (REC), and expressive (EXP). The Prev and Next buttons allow us to visit other domains.