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Are Field Sobriety Tests Accurate?

Before we delve into the scientific basis—or, more accurately, the lack of scientific basis—of Field Sobriety Tests (FSTs), let’s consider how much depends on the subjective judgment of the police officer. Whether a person is arrested, whether the district attorney decides to press charges and whether a person’s driver’s license is revoked at the DMV hearing is all largely dependant on the police officer’s judgment. The likelihood of a conviction is also strongly influenced by the police officer’s testimony, as well as the contents of the police report.

How well a person does on a FST is an important part of the equation. In theory, a person’s performance on an FST could mean the difference between being let go and being arrested. For the whole process to be reasonable and fair, we would hope that, by and large, police officers are good at judging the results of these tests. And this is what most people assume. Most people assume that if someone failed a FST according to a police officer’s judgment, the person was almost certainly drunk. This assumption is understandable given the stereotypical scenario that most people have in their heads: a guy who, when asked to walk a line, wobbled along before falling flat on his face (hiccupping the whole time). Even if we concede that reality is rarely so cut and dry, we would still like to think that police officers should be able to tell—maybe not 100% of the time, but at least most of the time—whether a person is drunk or not based on their performance.

Unfortunately, it’s just not the case. Dr. Spurgeon Cole of Clemson University decided to do an experiment to see how good police officers were at distinguishing someone who is under the legal limit from someone who is too drunk to drive, based entirely on watching them perform field sobriety tests. 14 police officers were shown videotapes of 21 subjects taking six common field sobriety tests and were asked to decide which “had too much to drink and drive.” On average, the police officers determined that 46% of the subjects were legally intoxicated.

So how did they do? Not well, especially considering that not a single subject had consumed alcohol. In other words, the blood alcohol level of every subject was .00%—as sober as you can get. The officers might as well have guessed randomly. This is a particularly disquieting result considering that, if the officers and pulled these individuals over, they would have arrested an innocent person half of the time. (Cole and Nowaczyk, “Field Sobriety Tests: Are they Designed for Failure?”, 79 Perceptual and Motor Skills Journal 99, 1994.)

Okay, so police officers aren’t well trained on assessing the results of field sobriety tests. But what about the tests themselves? Maybe the tests are perfectly valid and scientific.

Wrong again. Field sobriety tests have little to no scientific basis. Here’s a quick history of the modern field sobriety test. In the late 1970’s the federal government gave a grant to a research group called the Southern California Research Institute (SCRI) to come up with a series of field sobriety tests that were more reliable that the ones being used at the time. The tests that the group eventually came up with, by their own admission, were still far from perfect. The groups own data showed that roughly half of subjects tested would have been arrested, despite their BAC being under the legal limit. Unsatisfied with these results, the federal government gave SCRI another crack at it. In 1981 they came up with some better data. This time roughly 30% of subjects would have been falsely arrested.

Since 1981, SCRI has done numerous other studies and the barely passable 32% has been brought up to a confidence-inspiring 91%. Is this because these tests have been refined and “standardized”? While this is certainly what they claim, a careful examination of the actual studies that yielded these fantastic results paints a very different picture. A few researchers obtained their data and experimental design through the Freedom of Information Act and made some startling discoveries. (Hlastala, Polissar and Oberman, “Statistical Evaluation of Standardized Field Sobriety Tests”, 50(3) Journal of Forensic Sciences 1, May 2005)

For one, a large proportion of the subjects had blood alcohol levels so far over the legal limit that their performance on FSTs was nearly irrelevant. Second—and it’s almost too hard to believe—the police officers that were part of the experiment also had portable breathalyzers. In other words, when asked to estimate BAC from subjects’ performance on field sobriety tests, they were allowed to take into account breathalyzer results!

What the researchers also found was that the police officers were much more likely to overestimate a person’s BAC on the basis of field sobriety tests than underestimate it. Put another way, field sobriety tests made subjects appear more intoxicated that they actually were in the minds of the police officers. While it might sound unfair or exaggerated, the legitimate scientific studies on field sobriety tests point towards an unsettling conclusion: field sobriety tests are designed to make people fail.