Which countries around the world are currently at greatest risk of an onset of state-led mass killing? At the start of the year, I posted results from a wiki survey that asked this question. Now, here in heat-map form are the latest results from a rejiggered statistical process with the same target. You can find a dot plot of these data at the bottom of the post, and the data and code used to generate them are on GitHub.
These assessments represent the unweighted average of probabilistic forecasts from three separate models trained on country-year data covering the period 1960-2011. In all three models, the outcome of interest is the onset of an episode of state-led mass killing, defined as any episode in which the deliberate actions of state agents or other organizations kill at least 1,000 noncombatant civilians from a discrete group. The three models are:
- PITF/Harff. A logistic regression model approximating the structural model of genocide/politicide risk developed by Barbara Harff for the Political Instability Task Force (PITF). In its published form, the Harff model only applies to countries already experiencing civil war or adverse regime change and produces a single estimate of the risk of a genocide or politicide occurring at some time during that crisis. To build a version of the model that was more dynamic, I constructed an approximation of the PITF’s global model for forecasting political instability and use the natural log of the predicted probabilities it produces as an additional input to the Harff model. This approach mimics the one used by Harff and Ted Gurr in their ongoing application of the genocide/politicide model for risk assessment (see here).
- Elite Threat. A logistic regression model that uses the natural log of predicted probabilities from two other logistic regression models—one of civil-war onset, the other of coup attempts—as its only inputs. This model is meant to represent the argument put forth by Matt Krain, Ben Valentino, and others that states usually engage in mass killing in response to threats to ruling elites’ hold on power.
- Random Forest. A machine-learning technique (see here) applied to all of the variables used in the two previous models, plus a few others of possible relevance, using the ‘randomforest‘ package in R. A couple of parameters were tuned on the basis of a gridded comparison of forecast accuracy in 10-fold cross-validation.
The Random Forest proved to be the most accurate of the three models in stratified 10-fold cross-validation. The chart below is a kernel density plot of the areas under the ROC curve for the out-of-sample estimates from that cross-validation drill. As the chart shows, the average AUC for the Random Forest was in the low 0.80s, compared with the high 0.70s for the PITF/Harff and Elite Threat models. As expected, the average of the forecasts from all three performed even better than the best single model, albeit not by much. These out-of-sample accuracy rates aren’t mind blowing, but they aren’t bad either, and they are as good or better than many of the ones I’ve seen from similar efforts to anticipate the onset of rare political crises in countries worldwide.
The decision to use an unweighted average for the combined forecast might seem simplistic, but it’s actually a principled choice in this instance. When examples of the event of interest are hard to come by and we have reason to believe that the process generating those events may be changing over time, sticking with an unweighted average is a reasonable hedge against risks of over-fitting the ensemble to the idiosyncrasies of the test set used to tune it. For a longer discussion of this point, see pp. 7-8 in the last paper I wrote on this work and the paper by Andreas Graefe referenced therein.
Any close readers of my previous work on this topic over the past couple of years (see here and here) will notice that one model has been dropped from the last version of this ensemble, namely, the one proposed by Michael Colaresi and Sabine Carey in their 2008 article, “To Kill or To Protect” (here). As I was reworking my scripts to make regular updating easier (more on that below), I paid closer attention than I had before to the fact that the Colaresi and Carey model requires a measure of the size of state security forces that is missing for many country-years. In previous iterations, I had worked around that problem by using a categorical version of this variable that treated missingness as a separate category, but this time I noticed that there were fewer than 20 mass-killing onsets in country-years for which I had a valid observation of security-force size. With so few examples, we’re not going to get reliable estimates of any pattern connecting the two. As it happened, this model—which, to be fair to its authors, was not designed to be used as a forecasting device—was also by far the least accurate of the lot in 10-fold cross-validation. Putting two and two together, I decided to consign this one to the scrap heap for now. I still believe that measures of military forces could help us assess risks of mass killing, but we’re going to need more and better data to incorporate that idea into our multimodel ensemble.
The bigger and in some ways more novel change from previous iterations of this work concerns the unorthodox approach I’m now using to make the risk assessments as current as possible. All of the models used to generate these assessments were trained on country-year data, because that’s the only form in which most of the requisite data is produced. To mimic the eventual forecasting process, the inputs to those models are all lagged one year at the model-estimation stage—so, for example, data on risk factors from 1985 are compared with outcomes in 1986, 1986 inputs to 1987 outcomes, and so on.
If we stick rigidly to that structure at the forecasting stage, then I need data from 2013 to produce 2014 forecasts. Unfortunately, many of the sources for the measures used in these models won’t publish their 2013 data for at least a few more months. Faced with this problem, I could do something like what I aim to do with the coup forecasts I’ll be producing in the next few days—that is, only use data from sources that quickly and reliably update soon after the start of each year. Unfortunately again, though, the only way to do that would be to omit many of the variables most specific to the risk of mass atrocities—things like the occurrence of violent civil conflict or the political salience of elite ethnicity.
So now I’m trying something different. Instead of waiting until every last input has been updated for the previous year and they all neatly align in my rectangular data set, I am simply applying my algorithms to the most recent available observation of each input. It took some trial and error to write, but I now have an R script that automates this process at the country level by pulling the time series for each variable, omitting the missing values, reversing the series order, snipping off the observation at the start of that string, collecting those snippets in a new vector, and running that vector through the previously estimated model objects to get a forecast (see the section of this starting at line 284).
One implicit goal of this approach is to make it easier to jump to batch processing, where the forecasting engine routinely and automatically pings the data sources online and updates whenever any of the requisite inputs has changed. So, for example, when in a few months the vaunted Polity IV Project releases its 2013 update, my forecasting contraption would catch and ingest the new version and the forecasts would change accordingly. I now have scripts that can do the statistical part but am going to be leaning on other folks to automate the wider routine as part of the early-warning system I’m helping build for the U.S. Holocaust Memorial Museum’s Center for the Prevention of Genocide.
The big upside of this opportunistic approach to updating is that the risk assessments are always as current as possible, conditional on the limitations of the available data. The way I figure, when you don’t have information that’s as fresh as you’d like, use the freshest information you’ve got.
The downside of this approach is that it’s not clear exactly what the outputs from that process represent. Technically, a forecast is a probabilistic statement about the likelihood of a specific event during a specific time period. The outputs from this process are still probabilistic statements about the likelihood of a specific event, but they are no longer anchored to a specific time period. The probabilities mapped at the top of this post mostly use data from 2012, but the inputs for some variables for some cases are a little older, while the inputs for some of the dynamic variables (e.g., GDP growth rates and coup attempts) are essentially current. So are those outputs forecasts for 2013, or for 2014, or something else?
For now, I’m going with “something else” and am thinking of the outputs from this machinery as the most up-to-date statistical risk assessments I can produce, but not forecasts as such. That description will probably sound like fudging to most statisticians, but it’s meant to be an honest reflection of both the strengths and limitations of the underlying approach.
Any gear heads who’ve read this far, I’d really appreciate hearing your thoughts on this strategy and any ideas you might have on other ways to resolve this conundrum, or any other aspect of this forecasting process. As noted at the top, the data and code used to produce these estimates are posted online. This work is part of a soon-to-launch, public early-warning system, so we hope and expect that they will have some effect on policy and advocacy planning processes. Given that aim, it behooves us to do whatever we can to make them as accurate as possible, so I would very much welcome any suggestions on how to do or describe this better.
Finally and as promised, here is a dot plot of the estimates mapped above. Countries are shown in descending order by estimated risk. The gray dots mark the forecasts from the three component models, and the red dot marks the unweighted average.
PS. In preparation for a presentation on this work at an upcoming workshop, I made a new map of the current assessments that works better, I think, than the one at the top of this post. Instead of coloring by quintiles, this new version (below) groups cases into several bins that roughly represent doublings of risk: less than 1%, 1-2%, 2-4%, 4-8%, and 8-16%. This version more accurately shows that the vast majority of countries are at extremely low risk and more clearly shows variations in risk among the ones that are not.