There is quite a bit and that this phenomenon is happening is not controversial.  I turned this up quickly; you could probably find better sources with more careful research:  <a href="http:&#x2F;&#x2F;kff.org&#x2F;medicaid&#x2F;issue-brief&#x2F;tapping-nurse-practitioners-to-meet-rising-demand-for-primary-care&#x2F;" rel="nofollow">http:&#x2F;&#x2F;kff.org&#x2F;medicaid&#x2F;issue-brief&#x2F;tapping-nurse-practition...</a>