Of course the number of links factors into the equation, but Osborne says that she also focused on the increase in site traffic (as measured by an analytics suite) and metrics like
time spent on-site, number of pages viewed and bounce rate. Taking note of the sites that helped drive the traffic is also key, as having it picked up by influential bloggers and authoritative sites
in your niche is one thing--discovering that you have an underserved target market or demographic interested in your client is another.
Osborne added that measuring offline inquiries, conversions and of course, rankings for strategic keywords are also stats that can help quantify the value of a press release. And to be sure that the metrics can be directly attributed to the release--make sure that it includes specific keywords (or a product) that hasn't been advertised or optimized for.