Doc, What Are My Chances?

Introduction

When a physician has to deliver bad news to a patient, the bottom-line question is, "Doc, what are my chances?" The doctor's answers are then usually couched in the language of probability.

Recently, both patients and insurance companies have become more questioning, because theywantto understand the doctor's reasoning. What persists is an explanation gap, the distance between the doctor's best assessment and the patient's comprehension of both the probability and how it was obtained.

We seek to reveal the logic behind the medical diagnostic decisionmaking process and arm the patient with the appropriate vocabulary. Because decisions hinge on probabilities-"we'll proceed to this treatment if we can ascertain that there is at least a 50% chance you have the disease"- we demonstrate how the numbers are determined. This approach removes the fuzziness from the process, and the required arithmetic is not intimidating when taken one step at a time. To demonstrate the method, we work a sample problem from start to finish.

We illustrate how a graphical solution called a nomogram [Doerfler 2009] delivers numerical results easily. Moreover, nomograms can offer advantages over other computational methods; an appendix gives a comparison.