For example, astronomy degrees take five to six years on average, but observational astronomy degrees take six to seven due to limiting factors of weather, while theoretical astronomy degrees take five. Though there is substantial variation among universities, departments, and individuals, humanities and social science doctorates on average take somewhat longer to complete than natural science doctorates. These differences are due to the differing nature of research between the humanities and some social sciences and the natural sciences, and to the differing expectations of the discipline in coursework, languages and length of thesis. However, time required to complete a doctorate also varies according to the candidate's abilities and choice of research. Some students may also choose to remain in a program if they fail to win an academic position, particularly in disciplines with a tight job market; by remaining a student, they can retain access to libraries and university facilities, while also retaining an academic affiliation, which can be essential for conferences and job-searches.