A postdoctoral fellow, postdoctoral researcher, or simply postdoc, is a person professionally conducting research after the completion of their doctoral studies. Postdocs most commonly, but not always, have a temporary academic appointment during which time they are expected to professionalize by publishing papers and completing research, typically in preparation for a more permanent academic faculty position. According to data from the US National Science Foundation, the number of holders of PhD in biological sciences who end up in tenure track has consistently dropped from over 50% in 1973 to less than 20% in 2006. The decline in the proportion entering tenure-track careers occurred alongside a substantial expansion in doctoral education in the United States; the NSF reports a long-term upward trend in the number of research doctorates awarded since 1958, averaging 3.0% annual growth.