There are debates on measuring the extent to which investment in PR is value for money see here for a bibliography), however it largely seems to come down to one thing - column inches. Those investing see PR as simply media management and expect their PROs (in-house of agency) to get the brand name in the media. While some measurement schemes are sophisticated and employ semiotics to not just measure number but tone; it seems that the central edict is that any coverage is good.
Is it any wonder that PROs find it difficult to adapt their trade to politics. Most coverage is at best cynical and often two-sided. The political PR finds the national media sets the agenda, and the local media, particularly newspapers, can be similarly intractable. Hence there is a conundrum here. While most brands not facing a major crisis can probably get a story somewhere in the media, a political 'brand' cannot - hence measurement must be semiotic, comparing the actual briefing to the coverage and ensuring any bias began with the PRO. However, one cannot then claim success. A brand may get one bit of coverage, in one newspaper, in a corner of the rolling news schedule etc; a political brand may get a lot of coverage in one day and measurement will probably find that, on balance, the PRO has failed. So is there a role for PR for a political party?