Wednesday, October 7, 2015

Zero correlation between state homicide rate and state gun laws

Zero correlation between state homicide rate and state gun laws


The correlation between the homicide rate and Brady score in all 51 jurisdictions is +.032 (on a scale of -1 to +1), which means that states with more gun restrictions on average have very slightly higher homicide rates, though the tendency is so small as to be essentially zero. (If you omit the fatal gun accident rates, then the correlation would be +.065, which would make the more gun-restricting states look slightly worse; but again, the correlation would be small enough to be essentially zero, given all the other possible sources of variation.) If we use the National Journal data (adding the columns for each state, counting 1 for each dark blue, which refers to broad restrictions, 0.5 for each light blue, which refers to medium restrictions, and 0 for each grey, which refers to no or light restrictions), the results are similar: +0.017 or +0.051 if one omits the fatal gun accident rates.

Now of course this doesn’t prove that gun laws have no effect on total homicide rates. Correlation, especially between just two variables, doesn’t show causation.

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But since people have been talking about simple two-variable correlations between gun laws and crime, I thought it would be helpful to note this correlation — or, rather, absence of correlation.