The number of publications of Physical Review Letters per year and the number of citations of papers published in the same year. Note that this is not the total number of citations the journal received in year x, but the number of citations papers published in year x received in the subsequent years.
How are inpact factors calculated?
The impact factor (IF) is calculated by counting citations from peer-reviewed journals only.
extended IF also counts citations from books and conference papers. However, no patent, abstract, working papers, online documents, etc., are covered.
