Where can I find statistics on the average number of papers published while working on a PhD?
Byrnes (2007) found that for tenured faculty in top departments of psychology in the US there was a correlation between the publication rate during the first 7 years following a PhD and the number of publications during the PhD.


  individuals in the bottom quartile published an average of 1.03 predoctoral articles, those in the middle 50% and top 25% published 2.00 and 3.03 predoctoral articles, respectively


Note that the quartiles refer tot he number of articles published during the 7 years following the awarding of the PhD and not the number of predoctoral articles.

This study obviously includes only a small subset of all Psychologists (those who have gotten tenure at top Psychology departments), but may provide a hook into relevant literature.