The data was pulled from Voter Information Services reports showing how many votes each member of Congress missed in one year. A "missed" vote is a recorded vote on the floor of the House or the Senate during which the member "did  not make his/her position known." 

Votes during which the member "paired" for or against, voted present, etc. are NOT counted as "missed" votes.

FYI.  The reports do not show the actual content of the votes that were missed.