Any quantitative metric carries perverse incentives. My co-workers have a legend that backs this up, from the time before I joined. They had a quarterly bonus

Any quantitative metric carries perverse incentives. My co-workers have a legend that backs this up, from the time before I joined. They had a quarterly bonus system that would be adjusted for defect rate. If you had defects against your code, you'd lose on the bonus. Needless to say that this resulted in a "black market" for defects, with little gain in quality or productivity resulting.