This paper presents some extensions of theoretical neoclassical approaches to study and measure productivity growth and economic welfare. Qualitative changes that are introduced by investments in new capital goods are defined not only as an increase in the services obtained from capital stock but also as the "externalities" that the new goods produce within the economic environment. The proposed method is based on general behavioural model that takes into account the economic value of the impact of qualitative innovations on the consumers' economic welfare and the producers' flows of profits.