The development of well-functioning numeracy skills is a foundation for further mathematical skills. Mathematical skills develop in a cumulative fashion, and to help children establish these skills, we need to better understand how they in fact develop, how to identify children who are at risk of developing mathematical learning difficulties, and last but not least how we can help remedy these difficulties. To better understand and ultimately support these children, we need to apply different methods and research designs. We need to make sure we raise questions that are detailed enough in order to avoid brevity, and help to answer the major question which iswhich are the most important skills to help children who struggle in mathematics? Which particular skills might be more relevant to be assessed in order to identify children who are at risk of developing mathematical learning difficulties? The first study examined the developmental relationship between the approximate number system and early mathematical skills, in two different datasets. The main objective was to further investigate the theory that the approximate number system has a potential casual influence on mathematical development. First, we reanalyzed the dataset from a recent study by Elliott, Feigenson, Halberda, and Libertus (2019). Using cross-lagged panel model Elliott et al. (2019) claimed a reciprocal relationship between the approximate number system and early mathematics, however when reanalyzing this dataset by using a novel methodological approach, a random-intercept cross-lagged panel model, no evidence of a reciprocal relationship was found. Second, in a 1-year longitudinal study with three time points we examined the developmental relationship of the approximate number system and addition skills with the same methodological approach as in the reanalysis of Elliott et al.'s (2019) data. Here, the results did not show any evidence supporting a reciprocal relationship between the approximate number system and mathematical development either. Combined, this questions the idea that the approximate number system plays a vital role in the development of early mathematics, and vice versa. Moreover, this study displayed how different methodological approaches lead to different results. The second study is a validation study. The psychometric properties of the Early Numeracy Screener that was developed from a theoretical model, the core numerical skills model by Aunio and Räsänen (2016). The Early Numeracy Screener aims to detect children who are at risk of developing mathematical learning difficulties later on, and furthermore aims to identify three sets of early numeracy skills; namely counting skills, numerical relational skills, and basic arithmetic skills. Confirmatory factor analysis found evidence for a never getting tired of watching Harry Potter with me. To paraphrase Bess Streeter Aldrich, there is neither subtraction nor division in my love for you, only addition and multiplication.