The research can be conducted by first conducting an investigation of existing literature and statistics to determine the likelihood of each variable. Values can then be assigned to each variable to indicate the greatest to least likelihood.

Qualitative research can then be conducted regarding the greatest likelihood of each explanatory variable. This can be done by means of surveys, questionnaires and interviews to both criminology professionals, political leaders, and the general public.

The study design will then follow a sequence led by the specific explanatory variable chosen. Gun control, for example, can be used as the first explanatory variable to be confirmed or denied by the study results.

The selection of a variety of explanatory variables, as well as the various types of research method will be implemented to minimise selection bias. Furthermore, the focus on more than one type of variable as focus for the study also provides...

By taking into account more than one type of survey and data gathering method, the most likely explanatory variable for the crime rate range in the various states can be obtained.
Further study might include more than one state, or use states with median crime rates as a balancing factor for the findings obtained for the states with very high or very low crime rates. Such studies could have a wide range of implications for criminology professionals in the United States, as well as the effort to reduce violent crime.

This can also be applied among different countries, where the effect of gun control on political or religious violence can be investigated.

References

Johnson, J.B., Reynolds, H.T. And Mycoff, J.D. (2008). Political Science Research Methods. Sixth Edition. CQ Press

King, G., Keohane, R.O. And Verba, S. (1994). Designing Social Inquiry. Princeton University Press