A postdoctoral fellow, postdoctoral researcher, or simply postdoc, is a person professionally conducting research after the completion of their doctoral studies (typically a PhD). Postdocs most commonly, but not always, have a temporary academic appointment, sometimes in preparation for an 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. They continue their studies or carry out research and further increase expertise in a specialist subject, including integrating a team and acquiring novel skills and research methods. Postdoctoral research is often considered essential while advancing the scholarly mission of the host institution; it is expected to produce relevant publications in peer-reviewed academic journals or conferences. In some countries, postdoctoral research may lead to further formal qualifications or certification, while in other countries, it does not.