Basically we're using the national household survey and the census, so those are every five years, and the labour force survey collects occupation data on a monthly basis. In order to do this sort of study in which you're looking at both field of study at a very detailed level and occupation data at a very detailed level, you need the depth of information that you can get only from the national household survey, which is done every five years.
On March 26th, 2015. See this statement in context.