David Barker, head of communications at the British Heart Foundation, explains: "People give us their money and expect us to spend it effectively.
We have to be able to show how successful we have or have not been, so we can learn from it and plan for the future.
"Traditionally, the advertising industry has taken evaluation more seriously.
But it is being increasingly used in the voluntary sector, and the methods are becoming more sophisticated.
"If you spend large amounts of money on a strategy, you want to know what you are getting."
The BHF evaluates its media coverage monthly, on both a qualitative and quantative basis. For a smaller charity, however, such regular evaluation might be excessive. The Brooke, which relieves the suffering of working horses and donkeys in poor countries, was founded 70 years ago, but it has only just conducted its first communications evaluation - of its first national campaign, Courses for Horses.
The evaluation, which was carried out by the communications solutions agency Romeike, found that the media coverage generated by the campaign was worth up to £1.2m to the charity. "The campaign exceeded our targets," says Nikki Austin, the charity's press officer. "We expected to get four pieces of broadcast coverage, but we actually got 162. The use of celebrities also boosted our coverage, so our strategy is working."
The charity's evaluation allowed it to see which stories worked and which didn't, on both regional and national levels.
Austin adds: "Some of our supporters in Oxfordshire organised a Hobby Horse Grand National to raise money, but it also got us coverage in the regional press."
For charities that don't have a dedicated PR officer, this kind of evaluation might seem like a luxury they simply can't afford. But even the smallest charities can try to monitor their media coverage.
Barker says: "Admittedly, the British Heart Foundation can afford proper evaluation. But for smaller charities, even collecting your press cuttings is a start."