Private medical data for nearly 20,000 emergency room patients at California's prestigious Stanford Hospital were exposed to public view for nearly a year because a billing contractor's marketing agent sent the electronic spreadsheet to a job prospect as part of a skills test, the hospital and contractors confirmed this week. The applicant then sought help by unwittingly posting the confidential data on a tutoring Web site. In an e-mail sent to a victim of the breach, the billing contractor, Joe Anthony Reyna, president of Multi-Specialty Collection Services in Los Angeles, explained that his marketing vendor, Frank Corcino, had received the data directly from Stanford Hospital, converted it to a new spreadsheet and then forwarded it to a woman he was considering for a short-term job. The job applicant apparently was challenged to convert the spreadsheet -- which included names, admission dates, diagnosis codes and billing charges -- into a bar graph and charts, Stanford Hospital officials said.