We're always being asked to demonstrate what return on investment we can promise to clients - and from various conversations I've had with colleagues in the industry, we're not the only agency to be asked. If a client spends 5k with us, what will they make back?
It's an almost impossible question to answer - and trust me, I've tried as hard as anyone to come up with a metric that shows the direct impact of PR on sales. Note here that I'm talking about a direct link from PR to sales rather than the things we can track rather more easily (reputation, awareness, positioning etc).
The problem is that mostly we have no real control over the end product we're promoting. We can choose not to get involved, or we can spot problems and feed them into the R&D process - but ultimately, we rely on our clients to produce things that people want.
Recently, we've worked with a company that lets people compare the best prices and buy stuff from their mobile phones. It launched a very clever iPhone app, and we thought it might fly. We drafted the launch release, got some great photos, and talked to a lot of journalists. A half-page in The Sun later, and the company was top of the paid apps list on iTunes, with 30,000 people paying to download the app the first three days (it was then used 400,000 times in the first week). Did PR influence the sales? Absolutely.
Six months ago, we did something similar for another company, operating in the same sort of market. It too produced an iPhone app, we helped the company gets lots of exposure, and... nothing. Very little impact on sales. And yet, the coverage was just as good - the journalists and we all thought it was a decent product. Did PR influence sales? Not at all.
The difference was that people wanted the first product, and they didn't want the second one.
While media coverage is great at bringing a good product to the attention of potential customers, it won't persuade them to buy something they don't want; and it won't gloss over something that doesn't work. Which is why I'm yet to be convinced that it is possible to guarantee a direct sales return on a PR investment. I'd love to know what others think.