Click here for citation information.

The National Hospital Ambulatory Medical Care Survey (NHAMCS) is a national probability sample survey of visits to hospital outpatient departments (OPDs) and emergency departments (EDs) conducted by the National Center for Health Statistics (NCHS), Centers for Disease Control and Prevention. The survey is a component of the National Health Care Survey, which measures health care utilization across a variety of health care providers. The national estimates produced from these studies describe the utilization of hospital ambulatory medical care services in the U.S. The 1999 NHAMCS included a national probability sample of visits to the EDs and OPDs of noninstitutional general and short-stay hospitals, exclusive of Federal, military, and Veterans Administration hospitals, located in the 50 states and the District of Columbia. The 1999 NHAMCS included data collected from December 21, 1998 through December 19, 1999, and consisted of a sample of 489 hospitals.

The sampling frame for the 1999 NHAMCS was compiled from the hospitals listed on the April 1991 SMG Hospital Market Data Base. Hospitals with an average length of stay for all patients of less than 30 days (short-stay) or hospitals whose specialty was general (medical or surgical) or children’s general were eligible for the NHAMCS. Excluded were Federal hospitals, hospital units of institutions, and hospitals with less than six beds staffed for patient use. Within each hospital, either all outpatient clinics and emergency service areas (ESAs) or a sample of such units were selected. Clinics were in scope if ambulatory medical care was provided under the supervision of a physician and under the auspices of the hospital. Clinics were required to be “organized” in the sense that services were offered at established locations and schedules. Clinics where only ancillary services were provided or other settings in which physician services were not typically provided were out of scope. In addition, freestanding clinics were out of scope since they are included in the National Ambulatory Medical Care Survey, and ambulatory surgery centers (whether in hospitals or freestanding) were out of scope since they were to be included in the National Survey of Ambulatory Surgery which was first fielded in 1994.

The Bureau of the Census was the data collection agent for the 1999 NHAMCS. Census Headquarters staff were responsible for overseeing the data collection process, training the Census Regional Office staff, and writing the field manual. Regional Office staff were responsible for training the field representatives and monitoring hospital data collection activities.

About three months prior to the hospital’s assigned reporting period, NCHS sent a personally signed introductory letter from the Director of NCHS to the hospital administrator or chief executive officer of each sampled hospital. About one week after the mailing of the introductory letter, the Census field representative called the hospital administrator to arrange for an appointment to further explain the study and to verify hospital eligibility for the survey.

After the initial visit and the development of the sampling plan, the field representative contacted the hospital coordinator to arrange for induction of the sample ESAs and OPD clinics and for instruction of the hospital staff.

The actual visit sampling and data collection for the NHAMCS was primarily the responsibility of hospital staff. Hospital staff responsible for completing the Patient Record forms were instructed in how to complete each item by the field representatives. Separate instruction booklets for ESAs and OPD clinics were prepared and provided to guide hospital staff in this task. These booklets provided an overview of the survey, sampling instructions, instructions for completing the Patient Record forms, and definitions.

The field representative visited the sampled ESAs and clinics each week during the data collection period and maintained telephone contact with the hospital staff involved in the data collection effort.

The standard error is primarily a measure of the sampling variability that occurs by chance because only a sample is surveyed, rather than the entire universe. The relative standard error (RSE) of an estimate is obtained by dividing the standard error of the estimate by the estimate itself and is expressed as a percentage of the estimate. For the ED, the lowest reliable estimate for visits in the ED is 87,000 visits; for the OPD, it is 114,000. (Numbers lower than these have an RSE greater than 30%.) For the 1999 NHAMCS, approximate RSEs for estimated numbers of patient visits to the ED and OPD, respectively, were reported as follows: 10,000 visits, 86.9%, 94.8%; 20,000 visits, 61.6%, 67.5%; 50,000 visits, 39.1%, 43.5%; 100,000 visits, 27.9%, 31.7%; 200,000 visits, 20.1%, 23.8%; 500,000 visits, 13.3%, 17.3%; 1 million visits, 10.1%, 14.5%; 2 million visits, 8.1%, 12.9%; 5 million visits, 6.5%, 11.8%; 10 million visits, 5.9%, 11.4%; 20 million visits, 5.6%, 11.2%; 50 million visits, 5.4%, 11.1%; 100 million visits, 4.3%, 11.1%; 200 million visits, 4.3%, 11.0%.

Visit - A direct, personal exchange between a patient and a physician, or a staff member acting under a physician’s direction, for the purpose of seeking care and rendering health services. Visits solely for administrative purposes, such as payment of a bill, and visits in which no medical care was provided, such as visits to deliver a specimen, were out of scope.

Modes of arrival – The various modes of arrival are defined as follows:ambulance (arrives in public or private ambulance (via air or ground) that provides either Advanced Life Support or Basic Life Support), public service (arrives in a vehicle such as a police car, a social service vehicle, beach patrol, etc., or is escorted or carried by a public service official), walk-in (arrives by car, taxi, bus, or on foot).

Drug mention – The entry of a pharmaceutical agent ordered or provided (by any route of administration) for prevention, diagnosis, or treatment. Along with all new drugs, the hospital staff also records continued medication if the patient was specifically instructed during the visit to continue the medication.

Emergency department – Hospital facility for the provision of unscheduled outpatient services to patients whose conditions require immediate care and staffed 24 hours a day.

Emergency service area – Area within the ED where emergency services are provided. This includes services provided under the “hospital as landlord” arrangement in which the hospital rents space to a physician group.

Outpatient department – Hospital facility where non-urgent ambulatory medical care is provided under the supervision of a physician.

Clinic – Administrative unit within an organized OPD that provides ambulatory medical care under the supervision of a physician. Clinics are grouped into the following six specialty groups for purposes of systematic sampling and non-response adjustment: general medicine, surgery, pediatrics, obstetrics/gynecology, substance abuse, and other.

Disposition – Where the patient goes following an ED visit.