If you do a quick search on the internet to find out the accuracy of polygraph testing, you’ll find a wide range of figures – as low as 61% and as high as 100% – although most reputable sources will report somewhere between 85% and 95%. So why is there such a variation in the figures reported?
For one, the studies are conducted differently. Some polygraph testing methods have proven more effective than others, yet some studies include all types of approaches, even those that may be considered out-dated, in their results. Another factor that can affect the level of accuracy includes the level of training the polygraph examiner has received.
But one of the biggest reasons for the variation in the figures is how the results are calculated. Critics often classify inconclusive tests as “errors,” but an inconclusive result simply means that the examiner was not able to make a definite diagnosis, meaning that he or she could not say for certain whether or not the test subject was lying. Most often in these cases, a second polygraph exam is done at a later time.
To understand how this can affect the accuracy rates that are reported, consider this example: 10 polygraph exams are given, and the examiner is correct 7 times, is wrong 1 time, and has 2 inconclusive results. If you consider the 8 definitive results, 7 were correct, giving you an accuracy rate of 87.5%. But if you consider the inconclusive results to be errors, then the accuracy rate drops to 70% – with 10 exams and 7 correct decisions.
Here are a few figures from reputable studies to consider:
- The National Academy of Science’s 2003 analysis of the 30 most recent polygraph data sets found an overall accuracy of 85 percent, and their analysis of seven field studies involving specific incidents found a median accuracy of 89 percent.
- The Journal of General Psychology looked at 41 criminal cases in 1996 and found that control-question tests were 93 to 96% accurate.
- In a survey of police agencies in the United States, over 90% expressed either moderate or high confidence in their polygraph screening program, and 80% of them reported that in their experience the accuracy of the testing ranged between 86% – 100%.
Even though polygraph testing is very reliable, there are sometimes errors, and some people are concerned that those being examined can use methods to “beat” the test. There are many steps that polygraph examiners take to ensure that this is not the case, which I will discuss in my next blog.