This is an article where the results of a randomized trial on patient outcomes depending on the type of provider (NP vs. MD) are discussed. Over a 2 year-follow-up, patient outcomes (including patients' health status, satisfaction with care, and utilization of healthcare services (emergency department/urgent care, frequency of hospitalizations, and use of specialists' services)) has been shown to be comparable between 2 clusters of patients treated by physicians and nurse practitioners. Hence, this article provides evidence on the question about the quality of care that nurse practitioners provide in primary care settings.
http://www.ncbi.nlm.nih.gov/pubmed/15358970