https://chroniclevitae.com/news/445-in-hindsight-former-ph-d-students-reflect-on-why-they-jumped-ship

Interesting study by the Council of Graduate Schools (Ph.D. Completion Project). Out of 9,000 doctoral students they tracked from early nineties to 2004, 57% completed the program within 10 years, while 30% dropped out. Interestingly enough, there are two times where attritions spike: the first two years and when students begin their dissertation. A deeper exploration would be interesting.