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The 'Breathalyser Fallacy'

7 March 2011

A leading University of Ulster academic has warned of the dangers of relying solely on police breathalyser tests to convict motorists of driving under the influence (DUI).

Professor Vani Borooah says the sole use of breathalysers to determine whether a person is DUI is an example of a situation in which the outcome of a test – positive or negative – is used to determine whether a condition exists.

He refers to this as the “breathalyser fallacy”, explaining that it confuses the chances that a person would test positive if he/she was DUI, with the chances that a person was DUI if he/she tested positive.

As a comparative example he explains: “If someone has measles, the chance of them having spots on their face is almost 100 per cent.

“But if someone has spots on their face the chance of them having measles may be quite low – the spots may be caused by acne, insect bites etc.

“Almost all of the criticism of the conclusions of breathalyser tests – namely, anyone over the limit is DUI, the person below the limit is not – concerns the likelihood of an innocent driver being wrongly convicted.”

Professor Borooah, Professor of Applied Economics at the School of Economics, argues the point.

He says: “Providing the police have strong ‘a priori’ reasons for making a person undergo a breathalyser test (ie he/she offers clear signs of DUI) and the test has a high probability of showing that person to be over the limit, then the probability of a positive outcome on the test would be high.

“But there is also a significant chance that an innocent driver (one who is not DUI) will be incorrectly identified as being over the limit, because only the breathalyser results are taken into consideration.”

Professor Borooah’s research paper entitled “Driving Under the Influence: The Breathalyser Fallacy” has recently been published in the Applied Economics Letters.