In May 2015, U.S. congressman Sander Levin argued that it is difficult to enforce trade deals, as he questioned Vietnam's willingness to meet the labour standards of TPP. A report by U.S. Senator Elizabeth Warren said that there was a huge gap between the labor standards of past US free trade agreements and the actual enforcement of those provisions. However, PIIE analysts note that research shows that the presence of "sticks" (a possible suspension of trade benefits) and "carrots" (technical assistance) in trade deals increase the likelihood that labor obligations in trade deals have a positive effect; there are both sticks and carrots present in TPP.