Counter-Terrorism and Sentencing Bill (Second sitting)

Debate between Ruth Cadbury and Laura Trott

Committee stage & Committee Debate: 2nd sitting: House of Commons
Thursday 25th June 2020 Public Bill Committees

[Ruth Cadbury]

Q My last question was about non-English speakers.

Professor Grubin: We do test with interpreters, and they seem to work just as well. Again, it does take training for the examiner to know how to work with an interpreter, and the interpreter needs training as well. Certainly, security services in other countries use it with interpreters quite regularly.

[Laura Trott]

Q Professor, you talked about a failure rate of up to 20%. What drives that failure rate? Is it the fact that people are tricking the test or that the examiner is making a mistake? What are the drivers behind that 20%?

Professor Grubin: There is a range of reasons that people can give either false positives or false negatives. Apologies for not looking at you while I am answering. Sometimes it is because the test hasn’t been set out properly, the examinee hasn’t been prepared properly in the pre-test interview or the questions haven’t been formulated well, and so on.

The examinee may have some other experience that is close enough to the way the question is being asked to cause that sort of response. For example, there was a very good study carried out in Israel. I won’t go through all the details of it, but they were able to debrief afterwards as we were with police officers who were applying for promotion. There were two false positives. The ground troops knew that these two people had been telling the truth, but they were said to be lying. One of them had said that he had previously made an insurance claim in Israel. At that time, the insurance companies in Israel would test people making insurance claims to see if they were honest or not. He said that he was being honest, but he was told that he was lying. He couldn’t get that out of his mind during the test. That causes the cognitive processing we were talking about, and it made him respond in that way.

The other person was more interesting. The experiment itself was about a test that the examinees could cheat on. You would know if they cheated or not. The second police officer said that he cheated when he took the test, but there was something wrong when he took it and the examiners had him do the test a second time. When he did it the second time, he said, “I don’t think I had better cheat again,” so he did it honestly. When he was asked if he had cheated on the test the second time, he said that he was thinking about having cheated the first time, which is why he reacted as he did.

There are other reasons as well, but it is hard to explain without going into the details about how polygraph testing works. Basically, you are comparing the relevant questions that you are interested in with so-called comparison questions. If those comparison questions are not evocative enough to elicit a response when a person is telling the truth to the relevant question, or vice versa, when they are too hot and the person is much more concerned about that question than about the relevant one, you can also get mistakes on the test.

The final reason is that sometimes we just don’t know; it just happens.