Although they represent public employees and bargain with government employers over how to spend taxpayer resources, public sector unions are technically private organizations. They are not subject to laws and rules that aim to make government entities transparent to the public. Information about them is, therefore, relatively limited.
There are just a few sources of information that can be used to measure union membership rates: 1) national population surveys, such as the U.S. Census Bureau’s Community Population Survey; 2) federal financial reports, such as the 990 and LM-2 forms that labor unions must file; and 3) public records requests of government employers’ payroll information. These sources use different methods to collect information about public sector workers and their unions, and each has its strengths and weaknesses. These are discussed in detail in the following sections.