In the United States, the PhD degree is the highest academic degree awarded by universities in most fields of study. American students typically undergo a series of three phases in the course of their work toward the PhD degree. The first phase consists of coursework in the student's field of study and requires one to three years to complete. This often is followed by a preliminary, a comprehensive examination, or a series of cumulative examinations where the emphasis is on breadth rather than depth of knowledge. The student is often later required to pass oral and written examinations in the field of specialization within the discipline, and here, depth is emphasized. After the comprehensive examination the student is a "PhD Candidate", which is the academic equivalent of a master's degree. Some universities will award terminal Master's for students who drop out of the PhD program at this stage, and some, like Columbia University and the Albert Einstein College of Medicine, award a Master's en route to the PhD. Some PhD programs require the candidate to successfully complete requirements in pedagogy (taking courses on higher level teaching and teaching undergraduate courses) or applied science (e.g., clinical practice and predoctoral clinical internship in PhD programs in clinical, counseling, or school psychology).
Another two to eight years are usually required for the composition of a substantial and original contribution to human knowledge in the form of a written dissertation, which in the social sciences and humanities typically ranges from 50 to 450 pages. In many cases, depending on the discipline, a dissertation consists of a comprehensive literature review, an outline of methodology, and several chapters of scientific, social, historical, philosophical, or literary analysis. Typically, upon completion, the candidate undergoes an oral examination, sometimes public, by his or her supervisory committee with expertise in the given discipline.
Admission
There are 282 universities in the United States that award the PhD degree, and those universities vary widely in their criteria for admission, as well as the rigor of their academic programs. Typically, PhD programs require applicants to have a bachelor's degree in a relevant field (and, in many cases in the humanities, a master's degree), reasonably high grades, several letters of recommendation, relevant academic coursework, a cogent statement of interest in the field of study, and satisfactory performance on a graduate-level exam specified by the respective program (e.g., GRE, GMAT). Specific admissions criteria differ substantially according to university admissions policies and fields of study. Some programs in well-regarded research universities may have very low acceptance rates and require excellent performances on standardized tests and in undergraduate work, strong support in letters of recommendation, substantial research experience, and academically sophisticated samples of their writing (see, for example, eligibility requirements at Princeton University.)
The number of PhD diplomas awarded by US universities has risen nearly every year since 1957, according to data compiled by the US National Science Foundation. In 1957, US universities awarded 8,611 PhD diplomas; 20,403 in 1967; 31,716 in 1977; 32,365 in 1987; 42,538 in 1997; and 48,133 in 2007. Over this same period, the number of students attending university rose greatly, by about a factor of four. Grading of students in US universities has trended upward as well. In a study of 135 universities and colleges, Rojstaczer and Healy found that grade point distributions have undergone gradual but significant changes since the 1960s so that 'on average, A is now by far the most common grade awarded on American four year campuses.' The authors hypothesize that this phenomenon may be due to a general desire by schools and instructors to elicit positive evaluations from students; the high grades 'bear little relation to performance.'