The Field Sobriety (SFST) Validation Studies
There have been numerous studies conducted by the Southern California Research Institute (SCRI) to “validate" the Standardized Field Sobriety Tests. These SFST studies have been sponsored by NHTSA.
What was the purpose of the Standardized Field Sobriety Test validation studies? The validation studies were conducted to find screening tests for officers to use on the side of the road to assist them in making arrest decisions based on the standard of Probable Cause. By having “Standardized” tests that all officers use, it also would assist judges and juries with determining how much weight to give the evidence from the tests. SCRI wanted to see if these tests were able to correlate to a specific Blood Alcohol Concentration (BAC). These studies at no point have ever attempted to determine if impairment could be detected with the field sobriety tests.
What is a false positive? A false positive incorrectly indicates that a condition is present. So when looking at the studies, a false positive would be when the field sobriety tests indicate that a person was above the specified BAC, but a breath or blood test showed that they were actually below the specified BAC.
Now that the basics are covered, the first (2) studies that will be addressed are the most recent studies. After that the other studies will be covered, primarily to show that there has consistently been issues with false positives.
Validation of the Standardized Field Sobriety Test Battery at BACs Below 0.10 Percent (1998)
This study is the most important to understand because, this is the most recent field sobriety study. Officers are trained to testify to the reliability of the field sobriety tests based on this study. It is commonly referred to as the San Diego Study, since it occurred in San Diego. It was a field study and was validating the field sobriety tests for a BAC of 0.08 grams or more.
What does NHTSA say? They say that when the (3) field sobriety tests are combined, they are 91% reliable in determining that a person is 0.08 grams or more. Additionally NHTSA broke down each test and listed its individual reliability.
- Horizontal Gaze Nystagmus: (4) or more clues is 88% reliable at indicating a person is 0.08 grams or higher.
- Walk and Turn: (2) or more clues is 79% reliable at indicating a person is 0.08 grams or higher.
- One Leg Stand: (2) or more clues is 83% reliable at indicating a person is 0.08 grams or higher.
The issues arise when the false positives are reviewed.
- Horizontal Gaze Nystagmus: (4) or more clues occurred in 37% of the people under 0.08 grams.
- Walk and Turn: (2) or more clues occurred in 52% of the people under 0.08 grams.
- One Leg Stand: (2) or more clues occurred in 41% of the people under 0.08 grams.
Where do the high percentages of reliability that NHTSA reports come from? The majority (72%) of people involved in this SFST study were over 0.08 grams. This means that if officers arrested everyone in the study, they would still be 72% accurate in their decisions.
Two more interesting notes to consider:
- The average BAC of the 217 people arrested was 0.150 grams.
- The average BAC of the 80 people not arrested was below 0.050 grams.
This is important to note because, it is easier to make a correct decision the further away from 0.08 grams a person is. An example would be a person with a high BAC who is obviously intoxicated; it will be clearly evident to the officer that an arrest is the correct decision.
The Robustness of the Horizontal Gaze Nystagmus Test (2007)
This is a laboratory study that only involved Horizontal Gaze Nystagmus. The study was conducted to determine if changes in the administration of HGN would affect the results. There were (3) elements tested:
- Stimulus Speed
- Fast (1 second)
- Standard (2 seconds)
- Stimulus Height
- High (4 inches)
- Standard (2 inches)
- Low (0 inches)
- 3. Stimulus Distance
- Close (10 inches)
- Standard (12-15 inches)
- Far (20 inches)
Looking at the results of instances in which HGN was performed correctly, the false positive rate was 67%. Additionally, 65% of the people below a BAC of 0.05 grams had (4) clues or more. There was a person with (6) clues at a BAC of 0.029.
When the stimulus was held too high, the false positive rate was 91%.
How did NHTSA address the extremely high false positive rate? The standards were changed to lower the number of false positives that were noted. In all training material and the (2) most recent field validation studies, (4) or more clues correlated to a BAC of 0.08 grams or more.
In this study, (4) clues correlated to a BAC of 0.03 grams or more. This drastically lowered the published false positives.
Psychophysical Tests for DWI Arrest (1977)
This is the first study and was used to determine which tests were the most accurate and easily administered by officers in the field. The BAC that these field sobriety tests indicated was 0.100 grams or above. There were (6) tests that were involved in the study.
- Alcohol Gaze Nystagmus (Later changed to Horizontal Gaze Nystagmus)
- Walk and Turn
- One Leg Stand
- Tracing Exercise with pencil and paper
- Finger to Nose
- Finger count
At the end of this study, the (3) most reliable field sobriety tests were determined to be Alcohol Gaze Nystagmus, Walk and Turn and One Leg Stand.
The false positive rate in this study was 27%. This means that 27% of the people having a BAC of under 0.100 grams were determined to be over the level by the officers.
Development and Field Test of Psychophysical Tests for DWI Arrests (1981)
This study was again using a BAC of 0.100 grams. A portion was conducted in a laboratory environment while the other segment was conducted in the field. For the laboratory portion, volunteers were “dosed” to specific BAC levels. Some of the volunteers received a placebo dose, which means their BAC would have been 0.000 grams. The volunteers were brought in on (2) separate occasions.
According to NHTSA this study showed that the Standardized Field Sobriety Tests were 81% reliable in determining if a person was at or above 0.100 grams.
Only the placebo-dosed individuals will be looked at for the false positive rate in this study.
During the first testing period, 18% of the placebo-dosed individuals were determined to be impaired by the officers. During the second testing period, 10% were determined to be impaired. This is a high rate of false positives for people that had no alcohol in their system.
Field Evaluation of a Behavioral Test Battery for DWI (1983)
This study was again using a BAC of 0.100 grams. The study was only conducted in the field.
According to NHTSA this study showed that the Standardized Field Sobriety Tests were 83% reliable in determining if a person was at or above 0.100 grams.
Of the incorrect decisions made by officers, 94% were incorrectly arresting someone whose BAC ended up being below 0.100 grams.
Field Evaluation Study of the Standardized Field Sobriety Test (SFST) Battery (1995)
This study is commonly referred to as the Colorado study and was a field study only. The BAC level involved in this study was 0.100 grams, but it was not considered a false positive unless the BAC was below 0.050 grams.
The average BAC of those arrested was 0.152 grams.
The false positive rate was 24%.
Field Evaluation Study of the Standardized Field Sobriety Test (SFST) Battery (1997)
This study is commonly referred to as the Florida Study and it was a field study only. This was the first study dealing with a BAC of 0.080 grams.
The average BAC of those arrested was 0.150 grams, and the average BAC of those correctly released was 0.031 grams.
The false positive rate was 18%.
As can be seen, the false positive rate remained high and consistent in all of the studies. The Standardized Field Sobriety Tests, especially HGN, have been given a lot of weight in court. Officers testify to the reliability of the field sobriety tests based on the NHTSA data, but the rest of the story is never heard.
Does this mean the SFST's are useless and should be eliminated? No, it means that the tests should continue to be used for the purpose they were created. To assist officers in "screening" drivers and determining whether or not to arrest the driver based on the standard of probable cause. The false positives come into play when in a courtroom and the standard is beyond a reasonable doubt. Then a jury needs to hear about and understand that these tests due have false positives and can be wrong.
About the author: Joshua Ott was a police officer for more than 10 years in Roswell, Georgia. He has extensive experience dealing with impaired drivers. He was a Drug Recognition Expert Instructor and is a Standardized Field Sobriety Testing Instructor and has trained numerous officers over the course of his career. He now operates Caselock, Inc, which provides expert testimony and case reviews for cases involving alcohol and drug impairment. Visit his website at caselockinc.com.