The traditional individual path to a PhD/doctorate remains the most common in Germany. Here, a professor supervises a PhD/doctoral student, who works on his subject in consultation with the professor, but largely independently. This form of PhD/doctoral study offers a great deal of flexibility, but demands a high degree of personal initiative and responsibility. How long a traditional individual doctorate takes depends on your own time schedule – or on the duration of your work contract. Three to five years are normal.

Depending on the subject area of your thesis, your research can be conducted alone or together with other researchers. Doctoral candidates are also free to choose where they would like to conduct research – either at a university, non-university research organisation or in industry

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http://www.research-in-germany.de/info/phd-in-germany/51594/individual-doctorate.html