Let's Consider those Gun Violence Studies

In the search for real understanding of the impact of an armed public on gun violence mass shootings there are lots of anecdotal information. And many studies with value ranging from painfully obtuse to useful and data driven.

One of the most "popular" studies recently is from the Mother Jones Magazine in their politics section.  In an update article they claimed:

We identified and analyzed 62 of them, and one striking pattern in the data is this: In not a single case was the killing stopped by a civilian using a gun. 

From its original article after the Colorado theater MJM described their methodology, they included cases where:

The shooter took the lives of at least four people. An FBI crime classification report identifies an individual as a mass murderer—as opposed to a spree killer or a serial killer—if he kills four or more people in a single incident (not including himself), and typically in a single location.

Their methodology statement reveals a tremendous flaw in their study (hence I would deem obtuse), part of which is pointed out by a survivor of one they excluded. Another examination shows why this criteria for discarding some data points is so flawed.

Auditing Shooting Rampage Statistics
July 31st, 2012   Submitted by Davi Barker, which shows the significnce of that flaw

With 15 incidents stopped by police with a total of 217 dead that’s an average of about 14.29. With 17 incidents stopped by civilians and 45 dead that’s an average of 2.33.

So setting the data inclusion value at 4 or more, effectively eliminates most cases where an armed civilian stops the shooter.  Another common tactic used is to essentially limit armed civilian cases to only those where the civilian actually shot.  Implying there is no effect from seeing someone else positioned to oppose the shooters rampage. Another tactic is whenever a shooter stops or is stopped by the presence of an armed civilian, studies and the media try to question was there any effect by saying "we don't know if they would have continued".  A form of argument from silence.

First Significant Case Study

 The following study covers a significantly larger sampling and pays due attention to other factors, including state laws and their changes.  We have 50 states essentially providing 50 experiments on laws, effects, and timing.

A paper published Feb, 27, 1999
Multiple Victim Public Shootings, Bombings, and Right-to-Carry Concealed Handgun Laws: Contrasting Private and Public Law Enforcement
John R. Lott, Jr. and William M. Landes
http://papers.ssrn.com/sol3/papers.cfm?abstract_id=161637
Page 6

We analyze multiple victim public shootings (hereafter, multiple shootings or killings) in the United States in the period 1977 to 1995. The main advantage of restricting our study to U.S. data is that we can compare states with and without shall issue laws at different points in time (other things constant) and, therefore, can estimate the effects of a change in the law within a state during the sample period.

page 17

Although one can also imagine circumstances where shall issue laws increase the availability of guns to potential offenders or where guns used in self defense lead to more not less killings, our results so far strongly indicate that these effects, if they exist, are not sufficient to offset the overall negative impact of shall issue laws on multiple shootings.
Here we separate the effects these laws have on the number of shootings from the number of people harmed. Suppose, for example, perpetrators are undeterred by legal penalties or the prospect of encountering an armed defender. Then the number of persons harmed per shootings could still fall (as the two school shooting examples suggest) if concealed handguns interfered with the offender's ability to carry out his plans. Using either the dummy law variable or the before-and-after time trends, the coefficients in Table 10 indicate that concealed handguns reduce both the number of shootings and the number of people harmed. The evidence on whether shall issue laws have a bigger impact on the number of people harmed relative to the number of shootings is mixed.

page 19

Again the percent of the population covered by shall issue laws continues to have negative and statistically significant effects on the number and change in the number of monthly shootings.
While the evidence provides little support for the copycat hypothesis, we note that our data contains shootings by adults. The recent public school shootings involving children might be different and more consistent with the imitative hypothesis. However, data on school shootings involve such a small sample that it is not possible to study these shootings separately.

Page 20

The second stage regressions support our earlier results contained in Table 6. Adopting a shall issue law is associated with a significant decline in the combined number of multiple killings and injuries (both absolutely and per 100,000 persons). In the separate murder and injury regressions, the coefficients are always negative and either significant or marginally significant (a t-statistic greater that 1.65).

Page 21

Finally, because the presence of citizens with concealed handguns may be able to stop attacks before the police are able to arrive, our data also allows us to provide the first evidence on the reduction in severity of those crimes that still take place.

Taking the data from their tables, I prepared the following charts to show the data trends for mass shootings. The Table 2 Chart shows statistically significant trends for fewer shootings in states with shall issue conceal carry.
The table 4 chart shows the statistically significant trend (the detailed mathematical treatment is described in detail in the PDF report) for fewer mass shootings once a state decides to become a shall issue state.  The trend indicates the probability the number of victims continues to decline the longer the state has been shall issue as well.

An Opposing Study

One of the later studies, June 2010, that reviews Lott, the National Research Council and others is found in this paper.
The Impact of Right-To-Carry Laws and the NRC Report:
Lessons for the Empirical Evaluation of Law and Policy
http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1632599

The debate on the impact of ―shall-issue or ―right-to-carry (RTC) concealed handgun laws on crime—which has now raged on for over a decade—is a prime example of the many difficulties and pitfalls that await those who try to use observational data to estimate the effects of controversial laws. John Lott and David Mustard initiated the "More Guns, Less Crime" discussion with their widely cited 1997 paper arguing that the adoption of RTC laws has played a major role in reducing violent crime. However, as Ayres and Donohue (2003a) note, Lott and Mustard’s period of analysis ended just before the extraordinary crime drop of the 1990s occurred. They concluded that extending Lott and Mustard’s dataset beyond 1992 undermined the ―more guns, less crime (MGLC) hypothesis. Other studies have raised further doubts about the claimed benefits of RTC laws (see for example, Black and Nagin 1997 and Ludwig 1998).
But even as the empirical support for the Lott and Mustard thesis was weakening, its political impact was growing. Legislators continued to cite this work in support of their votes on behalf of RTC laws, and the ―more guns, less crime claim has been invoked often in support of ensuring a personal right to have handguns under the Second Amendment. In the face of this scholarly and political ferment, in 2003, the National Research Council (NRC) convened a committee of top experts in criminology, statistics, and econometrics. Its purpose was to evaluate the existing data in hopes of reconciling the various methodologies and findings concerning the relationship between firearms and violence, of which the impact of RTC laws was a single, but important, issue. With so much talent on board, it seemed reasonable to expect that the Committee would reach a decisive conclusion on this topic, and put the debate to rest.
The bulk of the NRC report on firearms, which was finally issued in 2005, was uncontroversial. The chapter on RTC laws was anything but that. Citing the extreme sensitivity of point estimates to various panel data model specifications, the NRC report not only did not narrow the domain of uncertainty about the effects of RTC laws, but may have broadened it. The NRC report concluded there was no reliable statistical support for the ―more guns, less crime‖ hypothesis. One dissenting panel member, though, argued that the majority's own panel data estimates revealed that RTC laws did in fact reduce the rate of murder. Conversely, a different member of the NRC panel went even further than the majority’s opinion by doubting that any econometric evaluation could illuminate the impact of RTC laws.
Given the esteemed academics on the panel and the sharply conflicting conclusions on both the substantive issue of the impact of RTC laws and the suitability of empirical methods for evaluating such legal interventions, a reassessment of the NRC’s approach and findings could be useful for those seeking to estimate the impact of various legal and policy regimes. A systematic review of the NRC's evidence also provides important lessons on the perils of using traditional panel data methods to elucidate the impact of legislation. To be clear, our intent is not to provide what the NRC panel could not—that is, the final word on how RTC laws impact crime. Rather, we show how fragile panel data evidence can be, and how a number of issues must be carefully considered when relying on these methods to study politically and socially explosive topics.

Page 12

Indeed, there may be simply too much that researchers do not know about the proper structure of econometric models of crime.

By the time one gets to their conclusions you feel you have been twisted in myriad directions, all to make you simply more confused:

Finally, despite our belief that the NRC’s analysis was imperfect in certain ways, we agree with the committee’s cautious final judgment on the effects of RTC laws: ―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.

Wrap Up

The big take away from all these studies is:
1) It is an extremely complex problem that needs to look at essentially far too many sociological, economic variables to be able to make any easy or clear conclusion either direction of the argument.  The anti-gun contingent regularly uses this argument to disregard cases that don't support their arguments, but often will not apply to their cases.
2) However the flip side of that is when progressive anti-gun activists try to say "clearly guns increase crime" they are guilty of gross naivete in data and analysis and in fact.

Final comments come from a man who has been a gun store owner, involved in SWAT training, and the busiest Utah Concealed Weapons instructor. http://larrycorreia.wordpress.com/2012/12/20/an-opinion-on-gun-control/

I have written for national publications on topics relating to gun law and use of force. I wrote for everything from the United States Concealed Carry Association to SWAT magazine. I was considered a subject matter expert at the state level, and on a few occasions was brought in to testify before the Utah State Legislature on the ramifications of proposed gun laws. I’ve argued with lawyers, professors, professional lobbyists, and once made a state rep cry.
...
And here is the nail in the coffin for Gun Free Zones. Over the last fifty years, with only one single exception (Gabby Giffords), every single mass shooting event with more than four casualties has taken place in a place where guns were supposedly not allowed.

The evidence does not support the progressive anti-gun activists arguments to any significant degree.  The frothy over the top arguments and ad hominem attacks (and there are far too many) used to attempt to justify their gun legislation are not only illogical, but unconstitutional.   And ultimately that will be the deciding factor.