BAKERSFIELD, Calif. -- Retail medical clinics can now be found in pharmacies, grocery stores and "big box" stores.

Services are usually provided by a nurse practitioner. These quick-care centers are a growing phenomenon but have also generated controversy over the quality of care.

The RAND Corporation has studied nearly 1,000 retail health clinics and found that most are easily accessible to one-third of the U.S. urban population.

According to the study, typical patients are young adults who do not have a regular health care provider. For vaccinations and the treatment of conditions such as sore throats or bronchitis, retail clinics provide less expensive care with the same quality as other medical facilities.

The study also found that about one-fifth of visits to a doctor's office and one-tenth of visits to an emergency room could be adequately treated by a retail clinic.

The researchers presented their findings at a Congressional hearing held today, but said further study is needed to understand the full impact of retail health clinics.