Today was the inaugural meeting of a software craftsmanship group at my company. One of our members had a strong concern about metrics for measuring improvement

Today was the inaugural meeting of a software craftsmanship group at my company. One of our members had a strong concern about metrics for measuring improvement. My response was that "improvement" or "getting better" is too nebulous a concept, so it depends on how you measure adherence to a set of values. Does anyone have a more concrete approach to metrics of improvement? Or a better way to articulate this concept?