Hi, I too faced this problem, because unless the process of evaluation is standardized, it leads to biased evaluation. I downloaded and modified a rubric which solved my problem. Am attaching the one I used. Feel free to modify it to suit your methodology if you find it useful.

As someone who also produces news content I think you can look at this in a few ways that inherently lead to non-standardization. I know, non-standardization is scary. 

The first is of these is simple simple - was it designed to have an impact (something need to be designed to be impactful to be impactful)? Did a single piece of content directly lead to a result?

I think very few pieces of content achieve this standard, most works (including news articles, plays, songs, commercials) are part of a larger body of work that all build on each other and collectively impact a community.  The impact of some of these works is in fact that they created a foundation for more work, as an example I think of the first and subsequent articles on Joseph McCarthy that lead to Murrow and See it Now. 

While a standardized measure would be ideal, there is sadly no single measure of "impact." "Impact" could be taking to the streets,  electing a single reform-minded politician or bringing a whole new party into power, as a few examples. On a non-political issue (if you can argue that any issue is in fact "non-political," impact could be the actual change that the work advocated or highlighted, since a biased, partisan news source would not directly advocate.  

Maybe try working backwards: Impact was ______ the identified steps to ______ are 1,2,3,4..... Did the work achieve or lay the foundation for ________.

If there was a single critical work (or a definitive piece or pieces) that lead to ______ you could even snowball backwards, look at the work(s) that fed into that critical work(s), that lead to the feeder work, into that, etc. And you can do that simply enough by references/links especially if news sources, if it is a video work you can look at what archival video clips are used. Also interviews with the work's producers. 

I hope this helps.