Missing number is a test on the web-based progress-monitoring platform Levumi (www.levumi.de; Jungjohann, DeVries, Gebhardt & Mühling, 2018). The test at level 5a consists of an item pool of 70 items. The test duration is currently 5 minutes (as of January 2020). On the Levumi platform, the tasks are selected randomly from the item pool during the test period, so that a large number of possible parallel test versions can be generated automatically. The appendix contains an exemplary paper and pencil test version. Table 1 is the Q-matrix for the missing number test at level 5a composed of 14 dichotomously scored items and hypothesized difficulty parameters (DP) based on the integrated theory of numerical development (Siegler, 2016). Siegler (2016) provides with the integrated theory of numerical development a combined model of the development of numerical competence. Within this theory, arithmetic follows from the expansion of the mental number line. Siegler's theory offers an explanation for a number of discoveries within research into the development of mathematical competence: size and distance effects, the learning of various mathematical operations and SNARC effects (spatial-numerical association of response codes).