Obviously, how you calculate the Return on Investment (ROI) your program achieves will depend on the assumptions and variables you use. Ideally, the ROI can be measured as: 

(return-cost)/cost

But how you measure “return” and “cost” can be complicated. For example, in some cases, return might be measured as:

• changes in salary of mentees before and after participation in the program;
• changes in skill or knowledge of mentees before and after participation in the program;
• changes in productivity before and after institution of the program;
• change in turnover rates before and after institution of the program;
• changes in diversity of workforce before and after institution of the program;
• savings on amounts spent on recruitment before and after institution of the program.

Of course, regardless of what you determine you’ll measure as a program return, there will always be the lingering issue of whether – or to what extent – the result is directly attributable to participation in the program, as well as the question of the impact of changing business conditions and changes in the economy. Determining the costs attributable to the program is probably a bit more straightforward, as it is similar to determining training expenses.