Union density is defined as the share of workers in the state who are represented by a union, including union members and other workers who are covered by a union contract, based on the variable “union” from EPI extracts of CPS-ORG microdata. Low-union-density states are the 17 states with the lowest average union densities from 2015–2019 (all less than 8%). Medium-union-density states are the 17 states (including D.C.) in the middle of the union-density rankings (with union densities ranging from 8.3% to 13.3%). High-union-density states are the 17 states with the highest average union densities from 2015–2019 (greater than or equal to 13.5%). See Table 1 for more detail about these groupings. We average union density data across 2015 to 2019 for each state to give a more accurate estimate of states’ typical unionization rates over time. We do not include data beyond 2019 in our averages, to avoid any potential distortions related to the 2020–2021 COVID-19 pandemic and ensuing recession. For each group, we calculate the average of the 17 states’ median incomes. Median income here refers to income from all sources, including nonwork income sources, however, for the large majority of people surveyed, most of their income comes from labor earnings.