From the summary:
A new report from CEPR shows that union membership increases the wages of all workers, with low-wage workers seeing the largest gains.
This report uses national data from 2003 to 2007 to show that unionization raises the wages of the typical low-wage worker (one in the 10th percentile) by 20.6 percent compared to 13.7 percent for the typical medium wage worker (one in the 50th percentile), 6.1 percent for the typical high-wage worker (one in the 90th percentile). The paper also produces results for the 50 states and the District of Columbia. Throughout the states, a similar pattern holds, with unionization raising the wages of the lowest-wage workers the most.