The aims of the proposed research are: 1) To assess the extent of small area variation in ambulatory care, including comparisons between urban and rural regions; 2) To determine the relationship of these variations with provider characteristics, including organizational structure and physician specialty; 3) To examine the relationship between extremes of variation and patient care process and outcome measures of quality and; 4) To further the development of methods relevant to primary/ambulatory health services research, including measures of case-mix, quality and "outcomes." These methods will be applicable to activities of public and private payers that are directed at improving care efficiency and quality. the proposed research will build on prior work of the investigators in primary care and quality of care, and will extend the development of a recently completed (NCHSR sponsored) project on ambulatory case-mix. This new time-oriented classification system will be used in this study to adjust for differences in patient case-mix. The study population for this project will include over 150,000 persons enrolled in the State of Maryland's Medicaid program during 1985-1987; accordingly, the State's health department will be an active collaborative partner in this effort. The major source of data for this project will be a unique person- oriented data set constructed by the investigators from Medicaid claims data. Through the State, existing data from birth certificates and PRO reviews of all hospitalizations will also be obtained. Reviews of about 3000 ambulatory medical records are also proposed as a source of data on quality and outcomes for six indicator conditions, and, when compared with information from claims data, will serve as a means of validating information obtained from the billing data. Thus, the significance of this project is both methodologic and substantive; the decisions made in the ambulatory sector have profound effects on the cost and quality of US health care, and their understanding will be even more important in the era of "managed care."