The Center chose 10 states for this investigation based on their notoriety and high level of spending by outside groups like nonprofits and state-level super PACs: Florida, Iowa, Louisiana, Michigan, Mississippi, Montana, North Carolina, Ohio, Oklahoma and Wisconsin. Twelve states do not elect their justices: Connecticut, Delaware, Hawaii, Maine, Massachusetts, New Hampshire, New Jersey, New York, Rhode Island, South Carolina, Vermont and Virginia. Five states did not have supreme court elections: California, Pennsylvania, Tennessee, Utah and Wyoming. Of the remaining 23 states, elections were not considered competitive enough for analysis or outside spending did not play a major role.

For the analysis, the Center examined independent expenditure, electioneering, contribution and expenditure records filed with state agencies; political advertising invoices or contracts posted online by television stations with the Federal Communications Commission; 990 forms filed with the Internal Revenue Service by non-profit organizations active in these races; and the IRS database of disclosures filed by 527 political organizations. For Michigan, the Center used FCC data compiled by the Michigan Campaign Finance Network to supplement the analysis of available data.

The data collected by the Center is necessarily incomplete. Several nonprofit organizations active in the examined races failed to disclose the full range of their spending or information about their donors with state and federal agencies. Only television stations in the top 50 markets are required to upload political advertising information, which is displayed online by the FCC. Although stations in smaller markets are free to voluntarily post the documents, the Center found that few, if any, do so. Our examination thus only captured spending in the largest markets. Additionally, the most recent 990 filings were not available for many of the groups mentioned in the story.