Not only is there no metric, but when we force artificial metrics onto programming teams, we are forcing them to pursue a goal other than the quality of the software itself. Any benefits derived from measurability will be eclipsed by the bad practices brought about by the metrics.

Take the two of the most common pseudo-metrics: lines-of-code, bugs-closed-per-month.

Lines-of-code encourages verbosity and complexity while discouraging code reuse. It also ignores essential code maintenance tasks that usually remove lines-of-code rather than adding them.

Bugs-closed-per-month encourages the creation of bugs in the first place. Not that anyone intentionally makes mistakes, but it lets the programmer get away with getting it less-right the first time. Even worse, it assures continued dividends from an error-prone implementation.

It would be useful to seek macroscopic measures of programmer productivity in our discipline as a whole. Let's just keep in mind the potential adverse effects of applying these metrics on the ground. Realistically, any effective macro metric will be abused my PHBs all over the world in managing their 10 man teams.