From the abstract:
This report uses published data from the National Labor Relations Board (NLRB) to update an index, first developed by Weiler (1983) in the Harvard Law Review and modified by LaLonde and Meltzer (1991) in the University of Chicago Law Review, of the probability that a pro-union worker will be fired in the course of a union election campaign. We use the more conservative LaLonde and Meltzer methodology and also make adjustments for the rise since the mid-1990s in “majority sign-up” or “card check” organizing campaigns.
Using the Weiler (1983) and LaLonde and Meltzer (1991) methodology, we find that in the 2000s workers were illegally fired in over 1-in-4 (26 percent) of union election campaigns, up sharply from about 16 percent in the late 1990s. In 2007, the most recent year for which data is available, 30 percent of union election campaigns had an illegal firing. Pro-union workers faced about a 2.3 percent chance of being illegally fired during the course of the campaign. Even after we adjust for the effect of the rise in majority sign-up organizing campaigns, pro-union workers in 2007 appeared to have a 1.8 percent chance of being illegally fired.