Lobbying data were collected from the Senate Office of Public Records. The totals reflect filings registered and posted between January 1, 1998, and October 19, 2006. Disclosure forms must be filed twice a year. If the amount reported does not exceed $10,000, the organization can report "less than $10,000" as the amount. The Center has treated those filings as $0 when calculating totals.

Because of inconsistencies in the filing process, it is often difficult to discern a certain total. In some cases, the amount of money an organization self-reports is exceeded by the amount reported by its outside lobbyists. If the amount reported by an organization as in-house lobbying exceeds the money reported by its outside lobbyists, the Center has chosen to report only the inside money. Since many organizations include outside contracts in their self-reported totals, this is done to avoid any double counting. However, if the total amount reported by outside lobbyists is higher than the self-reported amount, the outside total is displayed. For further explanation, see the methodology.