Death by Random Forest: this project is a horrible misapplication of machine learning. Truly appalling, when a false positive means death:
The NSA evaluates the SKYNET program using a subset of 100,000 randomly selected people (identified by their MSIDN/MSI pairs of their mobile phones), and a a known group of seven terrorists. The NSA then trained the learning algorithm by feeding it six of the terrorists and tasking SKYNET to find the seventh. This data provides the percentages for false positives in the slide above. “First, there are very few ‘known terrorists’ to use to train and test the model,” Ball said. “If they are using the same records to train the model as they are using to test the model, their assessment of the fit is completely bullshit. The usual practice is to hold some of the data out of the training process so that the test includes records the model has never seen before. Without this step, their classification fit assessment is ridiculously optimistic.” The reason is that the 100,000 citizens were selected at random, while the seven terrorists are from a known cluster. Under the random selection of a tiny subset of less than 0.1 percent of the total population, the density of the social graph of the citizens is massively reduced, while the “terrorist” cluster remains strongly interconnected. Scientifically-sound statistical analysis would have required the NSA to mix the terrorists into the population set before random selection of a subset—but this is not practical due to their tiny number. This may sound like a mere academic problem, but, Ball said, is in fact highly damaging to the quality of the results, and thus ultimately to the accuracy of the classification and assassination of people as “terrorists.” A quality evaluation is especially important in this case, as the random forest method is known to overfit its training sets, producing results that are overly optimistic. The NSA’s analysis thus does not provide a good indicator of the quality of the method.
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