A 2000 UK systematic review (York) found that water fluoridation was associated with a decreased proportion of children with cavities of 15% and with a decrease in decayed, missing, and filled primary teeth (average decreases was 2.25 teeth). The review found that the evidence was of moderate quality: few studies attempted to reduce observer bias, control for confounding factors, report variance measures, or use appropriate analysis. Although no major differences between natural and artificial fluoridation were apparent, the evidence was inadequate for a conclusion about any differences. A 2007 Australian systematic review used the same inclusion criteria as York's, plus one additional study. This did not affect the York conclusions. A 2011 European Commission systematic review based its efficacy on York's review conclusion. A 2015 Cochrane systematic review estimated a reduction in cavities when water fluoridation was used by children who had no access to other sources of fluoride to be 35% in baby teeth and 26% in permanent teeth. The evidence was of poor quality. A 2020 study in the "Journal of Political Economy" found that water fluoridation significantly improved dental health and labor market outcomes, but had non-significant effects on cognitive ability.