National Research Council & Office of Scientific and Engineering Personnel 
On Time to the Doctorate [PDF ebook] 
A Study of the Lengthening Time to Completion for Doctorates in Science and Engineering

A need exists for better models of what contributes to changes in the time that students take to complete doctorate degrees. Utilizing numerous data sources, On Time to the Doctorate presents a new model to explain changes in both total time to the doctorate and in the several components of time to the doctorate. It examines the lengthening time to the doctorate in many fields (chemistry; physics and astronomy; earth, atmospheric, and marine sciences; mathematical sciences; engineering; agricultural sciences; biological sciences; health sciences; psychology; economics; and "all other social sciences") and provides theoretical explanations for this lengthening time. This thorough and current analysis of time to the doctorate provides an up-to-date, extensive review of the literature and an exposition of data not available elsewhere.