Software process assessment is an essential activity for improving software development in an organization. It is very difficult to put together an efficient and effective improvement plan unless it is based on the results of a preparatory assessment. This will determine the current status of the organization's software process and will identify the areas or points that need improvement. There is a need for a rigorous method of assessment that encompasses the factors that affect software production. By rigorous we mean an evaluation based on Evaluation Theory. This theoretical foundation must assure that the evaluation carried out is comprehensive and reliable. Evaluation Theory, defined by Scriven and other authors, describes the components for each type of evaluation method. Six guidelines can be deduced by generalising these components, according to which any evaluation method should be developed: target; criteria; yardstick; assessment techniques; synthesis techniques, and evaluation process. In this paper, we present a software process assessment method based on Evaluation Theory. This theoretical foundation was one of the things that compelled us to reflect on the factors to be included in the evaluation. As a result, the proposed method jointly assesses four essential factors in software production: processes; technology resources; human resources, and organizational design. The aim behind this method is to provide a remedy for the main shortcomings of current software process assessment methods: partiality of the evaluated factors, because most centre on the assessment of management processes; and non-rigorousness of the evaluation processes. The applicability of the proposed method has been corroborated by means of an experimentation conducted at small and mediumsized Spanish businesses.