Empirical software engineering is concerned with measuring, or estimating, both the effort put into the software process and the quality of its product. We defend the idea that measuring process effort and product quality and establishing a relation between the two cannot be performed without a model of cognitive and collective activities involved in software design, and without measurement of these activities. This is the object of our field, i.e. Cognitive Ergonomics of design. After a brief presentation of its theoretical and methodological foundations, we will discuss a cognitive approach to design activities and its potential to provide new directions in ESE. Then we will present and discuss an illustration of the methodological directions we have proposed for the analysis and measurement of cognitive activities in the context of collective software design. The two situations analysed are technical review meetings, and Request For Comments-like procedures in Open Source Software design.