The announcement has come at the same time as the news that ImpACT has now signed up more than 50 of the biggest charities, including the 15 with the highest annual turnovers.
Metrica has initially been hired for a year and is paid for by a number of the coalition members, according to Alan Gosschalk, chair of ImpACT and director of fundraising at Shelter.
However, he declined to identify them or to say how much the Metrica evaluation would cost.
He said: "I decided to go only to a few that had a bit of money, because it was easier than trying to get money out of everyone.
"Until we do it, we don't know what we are going to get out of it - but Metrica will be looking at the volume and favourability of articles and key issues that are covered, such as fundraising methods.
"It will also be broken down geographically and should help form our future strategy."
One charity that declined to join the ImpACT coalition is World Vision UK. Phillip Spencer, marketing director of World Vision UK, said the charity felt that transparency was already one of its strong points.
"World Vision is very supportive of what ImpACT is doing," he said. "However, on this occasion we haven't signed up to it because, as a very large global organisation, we have stringent accountability measures in place already. Here in the UK, World Vision has limited resources and we need to maximise these resources in the field."