I think that is the most fuzzy part. Determining combined productivity, factoring the "improved" quality and the knowledge sharing and gain is a lot more diffic

I think that is the most fuzzy part. Determining combined productivity, factoring the "improved" quality and the knowledge sharing and gain is a lot more difficult than simply adding up the individual productivity of each programmer (whatever may the metric for measuring the productivity be), but in this case, not tending to factor in improved code quality, constant code review and higher probability of catching subtle bugs