There is no one answer to this question as the percentage of PhD dissertations that fail varies depending on the institution, field of study, and other factors. However, a recent study by the National Association of Schools and Colleges of Arts and Sciences found that the average rate of dissertation failure across all disciplines was 9%. This means that roughly 1 in 10 PhD students will have their dissertations rejected by their committee. While this may seem like a high number, it is important to remember that a dissertation is a very lengthy and complex document. It is not unusual for students to revise their dissertations multiple times before they are finally approved.