The Problem with Averages in Understanding Homicide in the United States – Redux

I have previously argued that the problem with averages, as summary statistics, is they can obscure significant underlying differences in the data producing the averages. Two very different distributions of data can result in the same average.

Imagine a city that has two neighborhoods and a city-level average of 100 homicides per year. That average is the same whether those homicides are distributed evenly between the two neighborhoods or are concentrated entirely in one of the two neighborhoods. The average as a summary statistic only tells part of the story.

This scenario is not entirely imaginary, either. In fact, we know that homicide is extremely concentrated demographically (e.g., among men) and geographically (e.g., in impoverished neighborhoods).

This morning I found another example of this problem with averages in my email inbox. The “Daily Bulletin” published by the newsadvocacy outlet, The Trace, contained the following summary of a research article:

23 states with “stand your ground” laws saw 8% to 11% increases in monthly rates of gun homicide over 16 years. Proponents of the laws that remove a person’s duty to retreat before using deadly force say they make the public safer by deterring would-be attackers, but a new study led by Michelle Degli Esposti of Oxford University and three colleagues found the laws actually correlate to more homicides overall, as well as more gun homicides specifically. The study compared 23 U.S. states that enacted SYG laws between 2000 and 2016, and 18 that did not have them in the same period (excluding the nine remaining states because they could not accurately be sorted into either group with sufficient data). They found large regional differences, with many SYG states in the South — including Alabama, Florida, Georgia, and Missouri — seeing increases in violent deaths rise by as much as 33.5 perfect, while similar laws in Arizona, Indiana, Michigan, Nevada, Oklahoma, Texas, and West Virginia were not associated with any difference. “These findings suggest that adoption of SYG laws across the U.S. was associated with increases in violent deaths, deaths that could potentially have been avoided,” the authors wrote.

The Trace Daily Bulletin, 23 February 2022

Even in this short summary, we see this problem quite glaringly. On the one hand:

[A] new study . . . found [Stand Your Ground] laws actually correlate to more homicides overall, as well as more gun homicides specifically.

On the other hand:

They found large regional differences, with many SYG states in the South — including Alabama, Florida, Georgia, and Missouri — seeing increases in violent deaths rise by as much as 33.5 perfect, while similar laws in Arizona, Indiana, Michigan, Nevada, Oklahoma, Texas, and West Virginia were not associated with any difference.

So, Stand Your Ground laws “correlate to more homicides” (including with firearms) nationally, but of 11 states mentioned, SYG laws in 7 “were not associated with any difference” in the homicide rate.

Even ignoring the “correlation is not causation” argument, if a casual factor only has an effect in 4 of 11 cases, it would not seem to be an independent causal factor in the outcome of interest. What else is going on in those 4 cases that are acting in concert with that causal factor that is not happening in the other 7 cases?

NOTE: I have not yet read the article and I make no claim about the accuracy of the research, one way or the other. My concern is how the findings are being spun by The Trace. If The Trace is accurately conveying the authors’ conclusion, then the authors, too, are thinking too simplistically about their findings.

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10 comments

  1. And, please let us remember that while all murders are homocides the reverse is not true. The SYG research needs to delineate between justifiable homocides and those that are not. As Clint Smith, master instructor and sage has noted, “some people just need killin’”.

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      • Prof. Yamane;

        First, my apologies for the misspelling in my original post. I was in a bit of a rush and blew right past those squiggly red lines!

        As to all violence being bad IMHO that depends, as I’m sure you’re aware. In my personal experience I’ve met a (very) few dedicated pacifists who have maintained they would not use any form of violence even to save their own life. While I have no understanding of this position they certainly have chosen to walk the walk. Others who intellectualize violence and have no personal experience of same would most likely change their tune were their nose to meet someone else’s fist with impetus and intent.

        My personal take on this subject was best summarized by one of my martial arts instructors:

        “If you assess that something bad is about to/going to happen, leave. If you’re followed, run. If you run and they catch you, self-defense is now your only option other than to take a beating and possibly lose your life.”

        If a guy in a bar tells you if he sees you still there in 5 minutes he’ll come over and deliver a beating, I take him at his word, and I’m gone. As a lifelong CCW permit holder I have no intention of participating in the monkey dance.

        For a very thoughtful and trenchant discussion on violence, please allow me to recommend “Meditations on Violence: A Comparison of Martial Arts Training and Real World Violence” by Rory Miller, a former prison guard and CERT (Cell Extraction Response Team) team member.

        Liked by 1 person

  2. So The Trace once again dissembles in the name of advocacy. So much for being a news service.

    “23 states with “stand your ground” laws saw 8% to 11% increases in monthly rates of gun homicide over 16 years.” is reminiscent of my math major friend’s book “How To Lie With Statistics”. Some states saw dramatic rises, and some saw nothing. And in most cases, the changes, when they occurred, did not happen synchronously with the enactment of the laws. So what else was going on, folks?

    Oh, and what about the homicide rates over time in the non-SYG states? What was the variability there? They don’t seem to have plotted the “control state” rates in Figure 1.

    I have to read the paper more carefully, but really. First I’m in the middle of working on e-bike legislation.

    Liked by 1 person

    • “So The Trace once again dissembles in the name of advocacy…”

      Nailed it. Unfortunately, the vast majority of people are not that critical in their processing of ‘news’ and swallow whatever is on the page, hook, line and sinker.

      The well-documented fact that 60% of gun deaths in the US are suicides and not homicides leaves most people incredulous. Then point out the fact that if we remove that 60% as a separate issue that needs to be addressed in other ways (i.e. mental health), the number of gun-related homicides in the US per capita is a much different picture than that which is frequently painted in the ‘news.’

      If you really want to baffle anti-2A activists, ask them to take a look at the FBI’s own publicly available records of homicides on an annual basis, broken down by the implement used. It is quickly obvious that more people are killed with blunt instruments than ALL homicides involving rifles, of which so-called “assault rifles” would be an even smaller sub-set.

      And so, if we are really concerned about the homicide rate and are trying to develop policy based on the actual numbers (rather than manufactured emotion and hyperbole), we should really be focused on addressing “blunt instrument” as a higher priority than rifles. But if you point this out, the cognitive dissonance is deafening amongst those who have come to believe that “assault rifles” are public enemy #1 and killing thousands of people every year.

      Liked by 1 person

      • It’s true that if they wanted the biggest bang for their buck that they would go after handguns rather than “assault rifles.” Maybe they could go after “assault pistols”? But that would be a losing political formula, so they go after the rifles that they think are more politically vulnerable.

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