This paper includes a causal-based modelling of software measurement processes in order to clarify the empirical reasoning in software engineering field. A first overview about existing causal network approaches shows the problems and possible benefits using these formal techniques in the software engineering area. The definition and extension of the causal modelling using causal networks helps to understand the relationships between the different software process artefacts and their causalities. The causal network based process model (CNPM) concept is based on the causal network idea of Pearl. The description of first applications of the CNPM approach for CMMI demonstrates the empirical reasoning of the software improvement processes in an explicit manner.