When someone says a variable is “too expensive” or “too difficult” to measure, we have to ask “Compared to what?” If the information value of the measurement is literally or virtually zero, of course, no measurement is justified. But if the measurement has any significant value, we must ask: “Is there any measurement method at all that can reduce uncertainty enough to justify the cost of the measurement?” Once we recognize the value of even partial uncertainty reduction, the answer is usually “Yes.”
