One of the most common questions we get on marketing measurement begins with "How would you measure ...?"
When I hear those trigger words, my mind immediately goes to the response question: "What would you do with the information if you had it?"
There are two reasons the response question is so important prior to answering the original question.
First, the answer to the response question will help me understand the extent to which your organization has developed a thoughtful (if executionally challenged) perspective on critical metrics, or if you're still in the mode of identifying the superset of all possible things that might be measured.
Second, the answer will tell me the relative importance of the particular piece of information you're looking for and give me some sense of the economic value of having better certainty of knowledge. If having the knowledge will improve your business outcomes just marginally or not at all, we can pretty quickly agree that it doesn't really matter how we'd measure it. Conversely, if the expected economic value of knowing is significant, many possible doors open in terms of collection avenues, since we can presumably allocate a fair amount of resources to acquiring the knowledge and still show a very positive return on that investment.
Third (yes, I know I said there were only two, but it's a blog so cut me some slack), if your answer to the response question doesn't properly anticipate how having the knowledge would impact current decision processes, it's a sign that we need to lay some organizational groundwork before we even ask for the resources to go get the knowledge.
As you might imagine, most people are initially stumped by the response question. But if you think about your hairiest, most formidable measurement challenges in the context of the economic value of knowing, you really begin to define your priorities for knowledge aggregation.
Everything can be reliably measured -- somehow. The critical parameters (as in most business pursuits) are how much time, money, and political capital you're prepared to spend to acquire the knowledge. You can't possibly know what your tolerances are until you have some clarity on the value of knowing.