Regarding the 7 steps to the perfect measurement program in “Measure What Matters” book , which step do you view as the most critical and why?

Regarding the 7 steps to the perfect measurement program in “Measure What Matters” book , which step do you view as the most critical and why?

Why is iterative measurement necessary?

What are 5 ways to measure ROI?Which way do you think is most and least difficult to measure and why?