Statistics are persuasive, so much so that people, organizations, and whole countries base some of their most important decisions on organized data（资料）. But there's a problem with that. Any set of statistics might have something lurking（ 潜伏） inside it, something that can turn the results completely upside（优势） down. For example, imagine you need to choose between two hospitals for an elderly（上了年纪的） relative surgery（外科）. Out of each hospital's last 1000 patients, 900 survived at hospital A, while only 800 survived at hospital B. So it looks like hospital A is the better choice.

  But before you make your decision, remember that not all patients arrive at the hospital with the same level of health. And if we divide each hospital's last 1000 patients, into those who arrived in good health and those who arrived in poor health, the picture starts to look very different. Hospital A had only 100 patients who arrived in poor health, of which 30 survived. But hospital B had 400, and they were able to save 210. So hospital B is the better choice for patients who arrive at hospital in poor health, with a survival（ 幸存） rate of 52.5%. And what if your relative's health is good when she arrives at the hospital?