Because we have excellent, excellent information on this from the practical experience of treating hundreds of thousands of patients with cancer. It just doesn't happen. But patients don't understand that emotionally so it's a matter of saying it once and then reinforcing it and reinforcing it. And it's also a very easy trap to fall into to assume that just because a patient is sitting there relatively quietly that they're not in pain. Actually there are good medical studies to show that we are notoriously inaccurate in estimating how much pain another individual has.
Doctors are inaccurate doing it, and even the patient's immediate family can be very inaccurate.
So the patient really is the expert on their own pain. Zero means no pain, and ten is the worst pain you can imagine. Now, no man with prostate cancer has had the experience of having a baby, but my point is that we try to put it in very practical terms so that patients can use that scale and internally calibrate it and relate to it. And by using the same scale over and over again with the patients, it's been shown that patients can very accurately measure the intensity of their pain and how much it improves with a therapeutic manipulation such as giving a medication. What if a patient does become empowered to the extent that that patient finds that the physician is not responding appropriately.