Citing personal communication from a paper Can (should?) I cite a personal communication from another paper? 
In my specific case, I want to express that a certain metric is not available yet, but it is the focus of ongoing research. I know this from the following section of Warton et al. 2012: 


  but it is the subject of ongoing research (M. J.Anderson, pers. comm.).


Do I just cite the Warton paper or use the approach suggested in 
Citing a Work that Cites a Personal Communication for tabular data?
The particular paper (Warton et al. 2012) the OP is talking about is 6 years old. After such time, the research it referred to may not necessarily be still ongoing - it is likely to be either completed, or abandoned altogether. It would be best to personally communicate with the person in charge of the mentioned research (i.e., M. J. Anderson in this case) in this matter. Either there will be a publication to refer to and cite (citing also the 2012 paper if it's still relevant), or one can just mention the personal communication like it was done by Warton et al.