David Seah has written an interesting article about measuring productivity and why. It’s a tough job because of the vague nature of productivity and how it affects our work.
Off the cuff, I’m thinking that STRENGTH here can be STRENGTH OF ATTENTION. In other words: FOCUS. The idea of CHALLENGE (that is, level of difficulty) might be measured by the NUMBER OF INTERACTIONS between the person and the work area that are occurring in the real world.
The simple idea, as David states, is just getting things done and not fretting about measuring your productivity. That said, if you were trying to get a better understanding of how your time and energy is spent, a unit of measurement for productivity may be worthwhile.
How to Measure Productivity Power? - [DavidSeah]