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researcher with the requisite skills and resources can conduct a systematic review, reviews undertaken by the Cochrane collaboration are often valued for their rigor and objectivity . The criteria used to select studies and rate their methodological quality are critical components of systematic reviews . Virtually all systematic reviews use a similar " hierarchy of study designs " when judging quality of the evidence : properly conducted randomized controlled trials are at the top of the hierarchy, observational studies are given lower quality ratings, and case reports or expert opinions are at the bottom of the hierarchy . Nonetheless, systematic reviews vary in the details and application of the evidence hierarchy . For example , a recent Cochrane review of water fluoridation applied GRADE criteria to evaluate quality of studies, commenting that "when applying GRADE to non - randomised studies, the quality of the evidence automatically starts at ' low', as opposed to ' high ' . " 43 In contrast, the U . S . Preventive Services Task Force considers a broader set of characteristics in rating the quality of evidence , particularly when evaluating preventive public health interventions for which randomized trials are not feasible .44 The first systematic review of fluoridation was conducted in 2000 by the United Kingdom ' s National Health Service Centre for Reviews and Dissemination .45 The research question was defined thus : "What are the effects of fluoridation of drinking water supplies on the incidence of caries" . While the search criteria placed no historical restrictions on studies to be reviewed , only 26 met the investigators' inclusion criteria which stipulated that studies had to report changes in caries over time, comparing one community where fluoride was added to or removed from drinking water with a control community that did not change its fluoridation status . Specifically, it required that communities be assessed at "two points in time, one of which is less than one year since the change of water fluoridation status in one of the groups" . The review, which included meta analysis to quantitatively combine the published results, concluded that introduction of fluoridation was effective in reducing dental caries, with the mean difference between communities in changes in dmft/ DMFT ranging from 0 . 5 to 4 . 4 teeth per child . The review also found that withdrawal of water fluoridation led to an increase in prevalence of caries . It noted that most of the included studies were of moderate quality ( level B ) . That rating was driven by the fact that there have been no randomized controlled trials of fluoride in drinking water for dental caries prevention . Instead , the 26 studies reviewed used observational study designs which ranked lower in the evidence hierarchy applied by the reviewers when evaluating quality . Other systematic reviews soon followed , although they varied in the number of studies reviewed . A 2007 review by Australia ' s National Health and Medical Research Council updated the U . K . review with two additional studies, concluding that introduction of fluoridation reduced cares while removal of fluoridation increased caries .46 In 2013 , the U . S . Community Preventive Services Taskforce updated the U . K . review with its own evaluation of 28 studies, endorsing fluoridation " based on strong evidence of effectiveness in reducing dental caries across populations . i47 It also found that fluoridation was effective in reducing caries across all socioeconomic groups . Meanwhile, a review of systematic reviews and practice guidelines concluded that water fluoridation is effective in reducing caries in children , with some evidence i48 that it can " reduce the oral health gap between social classes .