After successfully obtaining a "drs.", "ir." or "mr." degree, a student has the opportunity to follow a promotion study (informally called PhD) to eventually obtain a doctorate and subsequently the title "doctor". Promotion studies are structured ideally according to a preset time schedule of 4 to 6 years, during which the student has to be mentored by at least one professor. The promotion study has to be concluded with at least a scientific thesis, which has to be defended to "a gathering of his/her peers", in practice the board of the faculty with guest professors from other faculties and/or universities added. More and more common and in some disciplines even mandatory, is that the student writes and submits scientific publications to peer-reviewed journals, which eventually need to be accepted for publication. The number of publications is often debated and varies considerably between the various disciplines. However, in all disciplines the student is obligated to produce and publish a dissertation or thesis in book form.