Start the Discussion: Return On Learning Equation

In an earlier post Why Return on Learning is the better metric to use to justify elearning in the organization 
I stated that I would try and develop a ROL calculation so people could use it within their organization to justify using the free social media platforms for elearning purposes. The difficulty in conceptualizing and creating a new ROL calculation is that a good part of the benefits are hard to quantify, unlike in a ROI type equation. That said, here is my attempt and I would appreciate your feedback and thoughts.
Let's list a few potential inputs for the ROL equation:

• Retention and Achievement (thanks Richard)
• Higher employee satisfaction
• Productivity 
• Profitability 
• Engagement
• Innovation

Retention, profitability and innovation are inputs that can potentially be quantified while the others are more ambiguous. I think the above attributes are more desirable than a traditional ROI calculation because you can use these to measure employee development and company growth over time. A firm can track retention rates, profitability and even innovation. These items are more important for the growth of a company than a measure of an investment. Investment in people and a learning culture are much different than calculating the return on a capital investment. This point is key in understanding why a ROL type calculation is important to develop and use to track and employee development.
Before I go further, I would appreciate some insight into the inputs as well as any thing I should add.