Proponents and opponents have been both criticized for overstating the benefits or overstating the risks, and understating the other, respectively.[21][22] Systematic reviews have cited the lack of high quality research for the benefits and risks of water fluoridation and questions that are still unsettled.[14][22][23] A 2007 Nuffield Council on Bioethics report concluded that good evidence for or against water fluoridation is lacking.[46] Researchers who oppose the practice state this as well.[24] According to a 2013 Congressional Research Service report on fluoride in drinking water, these gaps in the fluoridation scientific literature fuel the controversy.[15] John Doull, chairman of the 2006 National Research Council committee report on fluoride in drinking water, has stated a similar conclusion regarding the source of the controversy: "In the scientific community, people tend to think this is settled. I mean, when the U.S. surgeon general comes out and says this is one of the 10 greatest achievements of the 20th century, that’s a hard hurdle to get over. But when we looked at the studies that have been done, we found that many of these questions are unsettled and we have much less information than we should, considering how long this [fluoridation] has been going on.