One percent of patients account for more than 25 percent of health care spending among the privately insured, according to a new study. Studies have already shown that Medicare spending is concentrated on a small group of individuals who are seriously ill. But an analysis by the IMS Institute for Healthcare Informatics, the research arm of IMS Health, a health information company in Danbury, Conn., provides a rare glimpse into the medical problems of people with private health insurance who are under 65. About three-quarters of them suffer from at least one chronic condition that could spiral out of control without proper care.