In a 2015 study conducted by the Law Center to Prevent Gun Violence, state gun laws were examined based on various policy approaches, and were scored on grade-based and ranked scales. States were rated positively for having passed stricter measures and stronger gun laws. Positive points were also given for states that required background checks on all sales of firearms and that limited bulk firearms purchases, and that prohibited sales of assault weapons and large-capacity magazines, and that carried out stricter evaluations of applications for handgun concealed-carry licenses, especially in the context of prohibited domestic-violence offenders. Meanwhile, points were deducted from states with laws that expanded access to guns, or that allowed concealed carry in public areas (particularly schools and bars) without a permit, or that passed "Stand Your Ground Laws" — which remove the duty to retreat and instead allow people to shoot potential assailants. Eventually, states were graded indicating the overall strengths or weakness of their gun laws. The ten states with the strongest gun laws ranked from strongest starting with California, then New Jersey, Massachusetts, Connecticut, Hawaii, New York, Maryland, Illinois, Rhode Island and finally Michigan. The states with weakest gun laws were ranked as follows: South Dakota, Arizona, Mississippi, Vermont, Louisiana, Montana, Wyoming, Kentucky, Kansas, and Oklahoma. A comparable study of state laws was also conducted in 2016. Based on these findings, The Law Center concluded that comprehensive gun laws reduce gun violence deaths, whereas weaker guns laws increase gun-related deaths. Furthermore, among different kinds of legislation, universal background checks were the most effective at reducing gun-related deaths.