The data comes from Peace Corps Open Government Initiative website. At the bottom of that page is a link to .csv files, including one for "2009 Peace Corps Volunteer on board strength - by post and sector" also available here. This page uses that data in its computations.
DATA AS OF 2009
Select a sector and region to see the number, and percentage, of volunteers within that region with that job sector
Example: 29% of 'Education' volunteers in 'Asia' were in China (in 2009) (In 2009 there were 383 Education volunteers in Asia of which 110 volunteers (29%) were in China)
This can be also be used as a GUIDE to where future Applicants may be placed
Data is based an actual distribution of volunteers in 2009
Some countries listed may have been suspended or evacuated.
Some countries have been added, but are not reflected in the data
Applicants can use the Timeline page to possibility narrow down the countries