sentences:  A 2001 study in the "Journal of Political Economy" by University of Chicago economist Mark Duggan did robustness checks of Lott and Mustard's study and found that the findings of the Lott and Mustard study were inaccurate.In 2004, the National Academy of Sciences (NAS) National Research Council (NRC) conducted a review of current research and data on firearms and violent crime, including Lott's work, and concluded "that with the current evidence it is not possible to determine that there is a causal link between the passage of right-to-carry laws and crime rates.