Basically, it's part of the national household survey and part of that was in the long form census. Everybody was asked about their highest level of schooling and what field of study they took. That was across the board, for anything they'd taken. The responses were then classified according to what they reported. Then for the job they were doing in the reference week, they reported what their major duties were. That was all classified according to a standard classification and then comparisons were made across various occupations.
On March 26th, 2015. See this statement in context.