Abstract
Today, a vast number of tools exist to measure development in early childhood in a variety of domains such as cognition, language, or motor, cognition. These tools vary in different aspects. Either children are examined by a trained experimenter, or caregivers fill out questionnaires. The tools are applied in the controlled setting of a laboratory or in the children's natural environment. While these tools provide a detailed picture of the current state of children's development, they are at the same time subject to several constraints. Furthermore, the measurement of an individual child's change of different skills over time requires not only one measurement but high-density longitudinal assessments. These assessments are time-consuming, and the breadth of developmental domains assessed remains limited. In this paper, we present a novel tool to assess the development of skills in different domains, a smartphone-based developmental diary app (the kleineWeltentdecker App, henceforth referred to as the APP (The German expression "kleine Weltentdecker" can be translated as "young world explorers".)). By using the APP, caregivers can track changes in their children's skills during development. Here, we report the construction and validation of the questionnaires embedded in the APP as well as the technical details. Empirical validations with children of different age groups confirmed the robustness of the different measures implemented in the APP. In addition, we report preliminary findings, for example, on children's communicative development by using existing APP data. This substantiates the validity of the assessment. With the APP, we put a portable tool for the longitudinal documentation of individual children's development in every caregiver's pocket, worldwide.