Just a couple of thoughts. As opposed to ROI, we typically see "revenue per employee" as a measure of productivity. While not often segregated into job functions, you can easily do this. If your shop floor personnel are completely responsible for all of you revenue, i.e., they make everything you sell, then ROI could be easily considered as revenue divided by headcount cost. This number should be going up if you are becoming more efficient. However, to do this for any given employee or over the tenure of any given employee would be difficult at best.