In the 1960s and 1970s, the federal government instituted funding for and development of programs for the education of physician assistants (PAs). The government initiated this educational funding for family physicians, nurse practitioners (NPs), and PAs to increase the supply of health care providers available to meet the needs of underserved populations. These governmental programs began to support training in primary care, internal medicine, and nurse mid-wifery. 1 Since then, additional health care proposals have come before Congress acknowledging and responding to increased demand for health care services and providers.2
When the government began to distinguish physician from nonphysician providers in the late 1970s, payment issues were addressed with the 1977 Rural Health Clinic Services Act,3 which enabled rural clinics staffed by PAs and NPs to be eligible for Medicare and Medicaid payments. This federal policy was created partially in response to health care access problems for beneficiaries in underserved rural areas2 and partially to recognize that PAs' skills and practice structure were suitable for serving rural areas.3 Federal Medicare and Medicaid policy amendments followed this act, allowing PAs to provide certain services without physician supervision (as permitted under state law).2
The use of PAs in the US health care system has significantly increased in recent years. More than half (56%) of physicians in group practice and almost 40% of independently practicing physicians use PAs, NPs, or nurse specialists to extend their ability to provide care to more people.4 The number of PAs and NPs increased from 228,000 in 1995 to 384,000 in 2005.4 This increase is primarily a result of the increased demand for health care services—a demand that most likely will continue as nonphysician providers fill the gap where physician shortages exist.5
Although little research has been conducted on the tendency of nonphysician clinicians to care for low-income 
patients and to practice in rural communities, research has shown that nonphysician clinicians and family physicians are more likely to care for underserved patients than are physicians in other specialties.1 Underserved areas were defined as being rural; a vulnerable community (having a high number of minority and poor residents); and previously designated as a primary care shortage area in 1998. In general, a higher percentage of PAs than physicians practiced in underserved areas and cared for a large number of uninsured patients or those on Medicaid.1
Greater access to primary care is linked to health care cost savings overall and improved quality of care. Nonphysician practitioners are cost-effective not only clinical models and fees.2 The potential savings in labor costs to be gained by increasing the use of PAs and NPs was estimated in 26 primary care clinics in a group managed care organization.6 Primary care clinics with a greater number of practicing PAs had lower labor costs than did practices that used fewer nonphysician clinicians.6
PAs can increase the efficiency of health care in underserved populations and have proved be a cost-effective way to improve access to care. We tested the hypothesis that underserved (defined as low-income and rural) populations are more likely to be seen by PAs than by physicians in outpatient clinics.
METHODS
Data source The National Ambulatory Medical Care Survey (NAMCS) is a national survey conducted by the National Center for Health Statistics and the Centers for Disease Control and Prevention. The NAMCS began in 1973, has been conducted annually since 1989, and supplies ambulatory care data from a national sample of office-based physicians involved in direct patient care. Physicians in anesthesiology, pathology, and radiology are excluded from the survey. The NAMCS does not include home, nursing home, organizational (bill-paying, paperwork, etc), or hospital visits. The survey uses a multistage probability design that encompasses probability samples of primary sampling units (geographic segments), physician practices within primary sampling units, and patient visits within practices.
The first stage of the NAMCS design includes 112 primary sampling units within the United States and Canada. The second stage includes a sample of nonfederally-employed, office-based physicians selected randomly from a list maintained by the American Medical Association and the American Osteopathic Association. The third and final stage includes randomly selecting the office visits to the sample of physicians. To achieve this, the physician sample is divided into 52 subgroups of approximately equal size, each of which is randomly assigned a 1-week reporting period in the survey year. The physician then randomly selects visits to record during that week.
The actual data for the survey are collected by the physicians with the assistance from staff, as instructed by the field representative. Physicians record data on a patient record form, which includes the patient name and record number for organizational purposes; however, all data are de-identified prior to physician submission. The data are edited before central processing, and all ambiguous entries are either reclassified or recoded. Computer edits are subsequently performed to identify any inconsistencies.
The NAMCS data are adjusted using the following four components: inflation (weekly data are inflated by 52 to produce annual estimates); adjustment for nonresponse (data are adjusted to account for physicians who failed to participate in the study); a ratio adjustment (an adjustment within each of the specialties to make the data representative of physician specialty distribution); and weight smoothing (a technique that preserves the visit counts in each specialty by switching the extra from largest visits to smaller visits).