Dr. Mark Chassin ("cha" as In "CHAT"-sin) Is the co-author of a report on medical mistakes In our nation's hospitals. This week In response to that report, President Clinton ordered that all U.S. hospitals take measures to reduce their errors. The report, Issued by the Institute of Medicine late last year, shook up patients and practitioners alike. It estimated that between 44,000 and 98,000 Americans die each year as a result of medical errors. The study was based on two studies In three states: New York, Utah, and Colorado.