This book isn’t just about numbers. It’s about what happens to students, teachers and parents because leaders have misunderstood or misused numbers. Frankly, this book is really about people, and the things they do to each other using numbers badly. If we were writing about the vital signs of patients in hospitals, mismeasurement would be easier to document, and its consequences more visible. Nurses might give medication to patients in the wrong dosage. Patients at home might fail to take their pills or take them twice. Doctors might misdiagnose patients’ illnesses. Or a doctor might misunderstand the meaning of a lab test because he or she doesn’t understand the probability of a false positive or false negative result. Manufacturers of intubation equipment might not sterilize valves or tubing. Drug companies might misunderstand the meaning of their clinical trials. Doctors might discharge patients too quickly, only to see them return days later. In every case, someone is harmed by mismeasurement, and that harm causes someone to suffer. In the school world, harm caused by mismeasurement takes a form that is less visible, and less well documented. But it is no less real to those who feel the pain, and those who are close to them. The pain it causes will range from large to small. When a student’s mastery of math is higher than his teacher realizes, and he is retaught something he already knows well, that student may be frustrated, annoyed or resigned. His time will be wasted as he is retaught something he knows, and his capabilities underestimated. A different sort of pain results when a student is misidentified as an English learner despite having equivalent mastery as his monolingual peers. That student may get routed into English language development classes for three, four or five years until an enlightened teacher realizes the original error and corrects it. The cost to the student: missed classroom opportunities and the demoralization that may result from being misunderstood. The cost to the district: wasted instructional time….