(Reuters) - Wisconsin and several other states are considering measures to reduce the power of public sector unions.

The U.S. has low union membership rates among private sector workers but much higher rates in the public sector. Here are some key facts about unions in the United States:

* PRIVATE SECTOR: In the private sector, 6.9 percent of workers are union members and 7.7 percent are represented by unions. By sector, union membership in transportation and utilities is 21.8 percent with 23.2 percent represented; construction 13.1 percent members with 13.7 represented; and manufacturing 10.7 percent members with 11.6 percent represented.

* PUBLIC SECTOR: In the public sector, 36.2 percent of workers are union members and 40 percent are represented by a union. Among the job categories are teachers, police, firefighters, civil service at the state and local level, university employees and public hospital employees.

* PUBLIC SECTOR BY STATE: Rates of union membership among public employees vary widely by state. New York has the highest rate at 70.5 percent members and 72.9 percent represented. Mississippi has the lowest with 7.4 percent members and 11.2 percent represented. In Wisconsin, the center of national attention now because of the debate there, 46.6 percent are members and 49.6 percent covered by a collective bargaining agreement. Other major states: California 56.6 percent members and 59.6 percent represented; Ohio 43.1 percent members and 46.2 percent represented; 26.8 percent members and 32.1 percent represented; Florida 23.3 percent union members and 27.8 percent represented, and Texas 16.9 percent members and 21 percent represented.

*FEDERAL WORKERS: Most unions representing federal government workers do not have the right to bargain over wages and benefits. While certain federal employees are allowed to form a bargaining unit and are eligible for union representation, the issues they can negotiate are very limited. Federal employees also cannot strike while in some states and local communities workers are allowed to strike. President Barack Obama proposed, and Congress approved, a salary freeze for 2011 and 2012 for civilian federal employees, which was not negotiated with unions through collective bargaining.

(Sources: Bureau of Labor Statistics, Union Membership and Coverage Database on www.unionstats.com, Federal Labor Relations Authority)

(Reporting and Writing by Lauren Keiper, Editing by Greg McCune)