Is there a return on Public Relations Investment?

I'm often asked about the success rate for public relations activities. Does what we do get clients media coverage and give a significant return on investment? Where we have clients who let us work consistently on their behalf for a period of time (a minimum of six months) they can get consistent coverage in the media - written, print, online, television, radio, etc. This consistency of interviews and articles works to drive traffic to client websites; initiates phone calls; provokes requests for proposals. All leading to a significant return on investment. In addition to this return, public relations is also bringing the client credibility and establishing them as an industry expert.