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Jan Smit at the Centre for Strategy and Evaluation Services appears to demean and devalue the degree of PhD by asking: "Something that I do not understand, and that no one has been able to confirm or explain to me despite calls to several universities, is the difference between the different kinds of doctorates, for example a PhD, a DLitt, a DPhil, a DSc or a DBA, and so on" (Letters, 1 January).

Let me try, for the UK system at least. A DPhil is a PhD (for example, the University of Oxford awards DPhil degrees, whereas Cambridge and many other universities award a PhD). A DSc and DLitt come later in a career: such degrees are honorary degree awards to those who already hold a PhD or DPhil.

Gordon Joly, London.