Impact Factor: 2.37 for PubMed-Indexed Articles only in 2015
1. How was this calculated?
In 2015, there were 27 articles indexed in PubMed. Those articles were cited 64 times as of 10/28/2016. Citing Data Source was from Google Scholar.
Then impact factor for these 27 articles in 2015 is: 64/27 = 2.37.
2. Why only PubMed-indexed articles used for IF?
As you know, people usually search PubMed to cite literatures.
If the articles were not indexed in PubMed, they have low chance of being cited, which does not mean they have low impact.
So if all articles were pooled together for IF, it is unfair for PubMed-indexed articles and it is not accurate.