For this analysis, the Center for Public Integrity attempted to review spending by the nation’s most “politically active” trade groups. Trade organizations that spent at least $1 million on federal lobbying in 2012 — as determined by the Center for Responsive Politics — were included in the analysis. The total number of organizations examined was 144.

To determine the spending of these groups, researchers collected annual nonprofit tax forms filed for the years covering 2008 through 2012. The IRS requires the associations to list their top five contractors as long as the total of each contract exceeds $100,000.

The analysis does not capture all payments made to contractors. While some trade groups and other nonprofits have recently begun filing their 2013 tax forms, 2012 is the most recent year for which all of the associations’ tax forms are publicly available.

Additionally, trade groups often vaguely describe the services their top contractors provide as “professional fees” or “consulting.” Because many firms offer a wide range of services, it’s often unclear exactly what kind of work was done on the industry associations’ behalf.