Are field sobriety tests actually scientific?

The distinction Florida courts draw is worth knowing, because it's more useful than the general question.

The psychomotor exercises — walk-and-turn, one-leg stand, finger-to-nose — came out of validation research and have a standardized administration and scoring manual. They're treated as observations an officer can describe.

HGN is treated differently. Florida's Fourth District has held HGN is scientific evidence, and that it isn't admissible on lay testimony alone — the traditional predicates for scientific evidence have to be satisfied.

Why the standardization matters for the psychomotor exercises. The validation research assumed the exercises would be performed as the manual describes: a level, dry, non-slippery surface, adequate lighting, instructions given a particular way, and a person without conditions affecting balance.

Real stops happen on sloped shoulders, in the dark, on gravel, with traffic passing and emergency lights strobing. The manual's own guidance is that departing from the prescribed administration compromises the validity of the results.