Economist John Lott has argued that right-to-carry laws create a perception that more potential crime victims might be carrying firearms, and thus serve as a deterrent against crime. Lott's study has been criticized for not adequately controlling for other factors, including other state laws also enacted, such as Florida's laws requiring background checks and waiting period for handgun buyers. When Lott's data was re-analyzed by some researchers, the only statistically significant effect of concealed-carry laws found was an increase in assaults, with similar findings by Jens Ludwig. Lott and Mustard's 1997 study has also been criticized by Paul Rubin and Hashem Dezhbakhsh for inappropriately using a dummy variable; Rubin and Dezhbakhsh reported in a 2003 study that right-to-carry laws have much smaller and more inconsistent effects than those reported by Lott and Mustard, and that these effects are usually not crime-reducing. Since concealed-carry permits are only given to adults, Philip J. Cook suggested that analysis should focus on the relationship with adult and not juvenile gun incident rates. He found no statistically significant effect. A 2004 National Academy of Science survey of existing literature found that the data available "are too weak to support unambiguous conclusions" about the impact of right-to-carry laws on rates of violent crime. NAS suggested that new analytical approaches and datasets at the county or local level are needed to adequately evaluate the impact of right-to-carry laws. A 2014 study found that Arizona's SB 1108, which allowed adults in the state to concealed carry without a permit and without passing a training course, was associated with an increase in gun-related fatalities. A 2018 study by Charles Manski and John V. Pepper found that the apparent effects of RTC laws on crime rates depend significantly on the assumptions made in the analysis. A 2019 study found no statistically significant association between the liberalization of state level firearm carry legislation over the last 30 years and the rates of homicides or other violent crime.