Membership in Sigma Xi is by nomination only. Selected individuals who have shown potential as researchers are invited to join as associate members. Full membership is conferred upon those who have demonstrated noteworthy achievements in research. Sigma Xi interprets “noteworthy achievement” to include primary authorship of two papers. ("Paper" includes refereed journal articles, patents, or internal reports. An earned doctoral degree may be substituted for one paper.) The Society also considers career preparation, career path, quality of research and similar factors.