What aspect of field sobriety tests makes them subjective?

Field sobriety tests are inherently subjective primarily because they depend on the officer's interpretation of how the driver performs during the tests. Each officer may have different thresholds for what they consider signs of impairment, leading to varying interpretations based on personal judgment. This reliance on individual observation and interpretation means that two officers could arrive at different conclusions based on the same performance of a driver. The subjective nature of these evaluations can significantly influence the outcome of a DWI arrest, as it ties directly to the officer's perception of behavior rather than an objective measurement.

In contrast, the other options present characteristics that do not enhance the subjectivity of these tests. For example, if the tests were conducted in controlled environments, that would decrease variability and therefore enhance objectivity. Similarly, assessing only physical ability without involving a subjective assessment of judgment would lead to a more straightforward, objective result. Lastly, stating that the tests have standardized results means that there is a common framework to evaluate outcomes, which would reduce subjectivity rather than reinforce it.