In the field of research with which this book is concerned, measurement is the quantification of any multidimensional phenomenon so that it can be more easily read and thus interpreted in order to obtain an overview of a complex reality. One of the most important steps is to establish a link between the concept (global definition and delimitation of the object of measurement) and the observable reality (data). Measurement is therefore a kind of ‘redde rationem’ of abstract concepts, in which the researcher can establish a link between theory and practice on the one hand and assign a value (or score) to the phenomenon in space and time on the other. Once this is done, any abstract assumption can be measured. It turns out that theory and measurement are inevitably linked and that a theory is incomplete without measurement and vice versa. The assumption that every poorly defined phenomenon is also poorly measured always remains valid.