The 2013/2014 impact factor of the journal PLOS ONE is 3.534. The impact factor of PLOS ONE was 4.351 in 2009, 4.411 in 2010, 4.092 in 2011 and 3.73 in 2012. The decline of PLOS ONE's impact factor is due to the increasing number of articles published, claims Phil Davis in the scientific blog The Scholarly Kitchen.
The journal impact factor is a measure of a journal's scientific influence. A journal's impact factor for a given year is calculated by dividing the number of citations received during the two previous years by the total number of works published during that time period. Since PLOS ONE publishes tens of thousands of articles each year, this way of measuring the journal's scientific influence negatively affects its impact.
The impact factor of a journal is very important for researchers, as many universities and research institutions assess the work of researchers based on the impact factors of the journals in which their works have been published. Hence, researchers tend to choose high-impact journals rather than lower-impact journals for submitting their work for publication. As a result, a small number of journals receive a great number of submissions, causing a bottleneck in the revision process and a delay in publication.
Alternative measures of a journal's scientific influence include Eigenfactor, altmetrics and h-index.