For years, a lot of money, time, and energy have gone into a national campaign to discredit teachers unions by saying they protect bad teachers and do nothing to add value to our system of public education. Whole organizations have been created to advance this campaign, and prominent individuals have staked their national reputations on the belief that unions are not good for education.

However, a recent report reveals this common critique of teachers unions is based on “myths.” The report uses empirical data analysis to correct the record on the effects of unions on the teaching workforce and, in turn, on an important measure of student education attainment: high school dropout rates.

On three basic questions – whether unions protect bad teachers from being fired, harm the quality of the teacher workforce long term, and do little to advance student outcomes – the answers to all three were “no.”

The report, “The Myth of Unions’ Overprotection of Bad Teachers: Evidence from the District-Teacher Matched Panel Data on Teacher Turnover,” which is dated October 5, 2015 and barely surfaced online the following month, has gotten virtually no attention in media outlets despite its startlingly contrarian findings.

Nevertheless, the report has gotten a second life in progressive circles, likely due to concerns over the U.S. Supreme Court case Friedrichs v. the California Teachers Association that could significantly harm the finances of unions should the verdict (due in early 2016) go to the plaintiff. The report also seems relevant to more recent developments in Chicago, where teachers are poised to vote on whether or not to strike in early 2016.

The report’s author, Wellesley College economics professor Eunice S. Han, looked for empirical evidence of the effects of strong teacher unions from about 4,600 districts – a third of U.S. public school districts – which included approximately 37,200 teachers within 7,500 schools. On average, approximately 70 percent of public school teachers are unionized.

Han also examined the causal effect of weakening unionism in four states in particular – Indiana, Idaho, Tennessee, and Wisconsin – where restricting the collective bargaining of teachers in 2011 provided “a natural experiment” for confirming her national analysis.

Through her analysis, Han finds, “Unionism raises the dismissal of low-quality teachers.”

Part of the positive influence of unions on the teacher workforce derives from one of the features of unions that most irritate school reformers: the exercise of collective bargaining for better wages and benefits.

Unsurprisingly, Han’s analysis finds teachers in districts where collective bargaining is allowed are more likely to remain in teaching than teachers in districts where bargaining is prohibited.

But improved teacher retention is not the only positive outcome of collective bargaining. The higher salaries that unions demand through collective bargaining encourages school districts to carefully evaluate new teachers’ performances during probationary periods and weed out ineffective teachers, in order to avoid paying the even higher wages once these teachers receive tenure.

Perhaps more influential than collective bargaining, however, is the overall strength of unions in a given district. In Han’s findings, the increased presence of union representation in a school district, what she calls “union density,” appears to have even a stronger effect than salary levels on dismissal rates of teachers early in their probationary periods and helps retain more high-quality teachers in the long run.

Her statistical model ultimately predicts that the education quality will be higher in districts with stronger union density than in districts with weak unionism.

Because teacher retention and quality are not the only factors that influence student outcomes, Han also looked at the effects of unionism on an important education attainment benchmark: high school dropout rates.

What she finds is that in districts where teachers are legally able to negotiate salaries and benefits – through either collective bargaining or a less structured “meet-and-confer” process – dropout rate decline by 6 percent and 4.5 percent, respectively. “These effects,” Han writes, “are substantial, considering the standard deviation for dropout rates is about 11 percentage points.”

To further validate her analysis, Han looked at those four states where conservative lawmakers had successfully weakened unionism to identify any causal effects of their actions. What she finds is that legal roadblocks to collective bargaining and other negotiation tactics significantly lower teachers’ likelihood of being certified as “high quality teachers,” a federal designation, by 3 percent. Han concludes the legal change in these states “reduces teacher quality, as it decreases teacher salary, diminishing districts’ incentives to dismiss low-quality teachers and encouraging high-quality teachers to leave the teaching sector.”

The report strengthens the positions of teachers in Chicago should they vote to strike, should be considered in deliberations over the Friedrichs case, and must help shape ongoing debates about the importance of unions in the nation’s public education system.