The disaggregation of PR and advertising impact has always been something of a Holy Grail for the PR industry, but now that someone has made a real stab at isolating the PR effect, the main question is why didn't someone think of this earlier.
Manning Selvage and Lee's I to I Tracker model, while thoroughly researched and tested, isn't exactly rocket science. Possibly one of the key factors in its development is that its originator Claire Spencer is an ex-ad woman who experienced the struggle in the early 80s by the advertising industry to prove the validity of advertising recall as a measure.
The model proposed by MS&L doesn't completely iron out that thorny problem of the public's inability to differentiate an article and an ad, but the agency claims that the consistent sample size of those that can and the checks built in, make it a statistically valid proposition.
But possibly its greatest contribution will be its attempt to prove ROI by pointing directly to sales figures - from the painstaking factoring up of the PR aware audience and the link to household product data, the I to I may hopefully sound a death knell for the dreaded advertising value equivalent.