The Lie Detector Lies

Is there a “lie detector?” Most people have been lead to believe that this is what the polygraph is. But most scientists think the polygraph is less than scientific. Here’s a quick look at the From the website of the American Polygraph Association:

“A steady pace of improvements in techniques and technologies has allowed the polygraph to remain the gold standard among the methods available for verification of truthfulness. While other technologies come and go, the longevity of the polygraph is a testament to its power to uncover those who would deceive.”

Hmm… Was the longevity of the practice of bleeding medical patients a testament to its power to heal? Perhaps the longevity of some practices is unrelated to their effectiveness. Longevity isn’t a very strong argument to start with.

Professor Stephen E. Fienberg, Chairman, Committee to Review the Scientific Evidence on the Polygraph, National Academy of Sciences, has this to say about the polygraph:

“Polygraph testing has been the gold standard, but it’s obviously fool’s gold.”

The American Polygraph Association website says that the problem of accuracy is one of differing methods of measuring it. Critics, they say, “who often don’t understand polygraph testing, classify inconclusive test results as errors.” This isn’t an error, they say, but I imagine that if you are accused of murder, and you are innocent, you might want a more accurate result from a polygraph test than “inconclusive.”

They explain the problem: “If 10 polygraph examinations are administered and the examiner is correct in 7 decisions, wrong in 1 and has 2 inconclusive test results, we calculate the accuracy rate as 87.5% (8 definitive results, 7 of which were correct.) Critics of the polygraph technique would calculate the accuracy rate in this example as 70%, (10 examinations with 7 correct decisions.)” They may have a reasonable argument there in some ways.

On the other hand, what is more interesting, is that even in an argument from the biggest promoters of the polygraph, they use an example of 87.5% accuracy. That might sound good until you realize that of a 100 people tested in a murder case, about 10 innocent people would be found to be “lying.” And there would also be 20 “inconclusive” results, which might include both innocent people and murderers.

Looked at another way, of a 100 murderers, 10 would be found to be telling the truth, and 20 would have inconclusive results. Out of 100 murders, 30 would not be identified, according to the accuracy assumed in the example.

But let’s look at their method again, with a new example form another perspective – that of the innocent who have “inconclusive” results. Suppose 100 innocent people are questioned about a crime, and one is found to be telling the truth, while the other 99 tests are “inconclusive.” This would appear to be a relatively useless test, right? It correctly identifies just 1 out of 100 innocent people, leaving a cloud of suspicion over the other 99.

However, measuring the results the way the American Polygraph Association does, the accuracy would be 100%. Inconclusive tests are not supposed to be seen as errors, after all. Hmm… does a lie detector test that correctly identifies just 1 of 100 innocent people sound accurate? Not to those who are left with their innocence unconfirmed. For them it might seem like the lies are on the other end of those polygraph wires.

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