How does the scientific community measure how "good" or "great" a journal is? How do you determine the "impact" of an author's work? Should it be purely on the number of citations? Or is it a matter of the "quality" of the research? But then again, how do you measure "quality"?
These questions caused people to create methods on how to calculate the impact or "good-ness" of an article, journal, or author. These calculations and statistical methods are called metrics. Metrics are debated over, and over, and over. Most popular metrics include: number of citations, impact factor, and h-index. There are hundreds of other metrics available, some better defined than others. See the link below for more information about other available metrics.
One metric used is impact factor. According to Journal Citation Reports (JCR), the impact factor is a ratio focusing on original research.
Impact factor = # of citations to all items published in the past two years (divided by)
# of articles and reviews published over those past two years.
For example, if a journal has an impact factor of 2.5, this means in the indexed year each article published was cited on average 2.5 times in the previous two years.
Impact factor is used for journals only.
Journals have impact factor, authors have the h-index. h-index is somewhat complex calculation taking into account the total number of articles published by an author and the number of citations per article.
An author's h-index, or h, is equal to the number of total articles published, as long as every article has at least h number of citations.
h-index is used for authors only.