A new curriculum in primary science education for North-Rhine-Westphalia (Germany) has come into effect with the onset of school year 2008 / 2009. The new curriculum defines aspects of scientific inquiry competence students should have attained by the end of primary school. According to the curriculum these aspects should be among the entry competencies of German 5th graders (10 – 11 year old). The project’s main goal is to assess students’ scientific inquiry competence at the beginning of secondary school (grade 5). Therefore, a paper-and-pencil test was developed for measuring students’ achievement, whereas the project’s main focus was on developing tasks for measuring low-performers competence. The test was conducted with students from different types of German secondary schools. For item construction a three-dimensional model of competence consisting of the dimensions of complexity, cognitive processes and previous knowledge was used. Moreover, the SDDS-model by Klahr & Dunbar (Klahr, 2000) was used for the development of different types of experimental tasks. The main part of data analysis was conducted by using Rasch Analysis.