22 Oct. 2014
A PhD study can probably be completed within the allocated time, but sometimes a study can take a particular twist implying that multiple years of extension are required. This NRC news article implicitly refers to Rens's study on circumstances related to PhD delays.
Related Research Report
Van de Schoot, R., Yerkes, M.A., Mouw, J.M. & Sonneveld, H. (2013). What Took Them So Long? Explaining PhD Delays among Doctoral Candidates. PLoS One, 8(7): e68839.