Union membership reflects the approximate number of union members in each industry. We collected data on yearly individual union members as well as information on members' industry composition. While it may appear to be simple, union membership is an elusive concept.  Unions differ on whom they consider to be active members. For example, over time and across unions, they differ on whether they count striking workers, unemployed workers, disabled workers, retired workers, etc. Sometimes unions provide rough estimates of their membership rather than an exact count, and some unions may occasionally inflate reported membership for various reasons. We seek to count dues-paying members.

See Methodological Appendix for further details.