Where can I find statistics on the average number of papers published while working on a PhD? I'm interested in this, because I want to know if there is something like an average number that we might find.

I am looking for statistics on the number of papers published while working on a PhD, preferably by general field of study (e.g. Physics, History, etc., not "Quantum Electrodynamics") and the country where the PhD was completed. If possible, the duration of PhD and amount of course work should also be parameters.
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.