Eunice Han - By demanding higher salaries for teachers, unions give school districts a strong incentive to dismiss ineffective teachers before they get tenure. Highly unionized districts dismiss more bad teachers because it costs more to keep them. Using three different kinds of survey data from the National Center for Education Statistics, I confirmed that unionized districts dismiss more low-quality teachers than those with weak unions or no unions. Unionized districts also retain more high-quality teachers relative to district with weak unionism. No matter how and when I measured unionism I found that unions lowered teacher attrition. This is important because many studies have found that higher-quality teachers have a greater chance of leaving the profession. Since unionized districts dismiss more bad teachers while keeping more good teachers, we should expect to observe higher teacher quality in highly unionized districts than less-unionized districts — and this is exactly what I found. Highly unionized districts have more qualified teachers compared to districts with weak unionism.