Commonwealth v. Taylor, 209 A.3d 444

Defendant Sarah E. Taylor crashed her car after driving fast; her 18‑month‑old child in a car seat was uninjured. Taylor exhibited bloodshot eyes, slurred speech, confusion, and poor performance on two standardized field sobriety tests.
Officer testified the poor performance indicated impairment by drugs; Taylor admitted taking Adderall and Xanax but no blood test or chemical evidence of impairment was introduced.
Defense theory: poor performance resulted from a possible head injury from the crash (not drug impairment).
Defense called Dr. Lawrence Guzzardi, a medical toxicologist who had written, lectured, and testified about field sobriety testing and toxicology, but who had not been trained to administer the standardized tests.
Trial court sustained Commonwealth objections and excluded testimony that standardized field sobriety tests are validated only for alcohol and have not been validated to detect drug impairment; jury convicted on DUI (drug impairment) and endangering welfare of a child.
Superior Court held exclusion was erroneous and not harmless; vacated sentence and remanded for a new trial on both counts.

Issue	Commonwealth's Argument	Taylor's Argument	Held
Whether trial court erred by excluding expert testimony that standardized field sobriety tests were validated only for alcohol and not for drug impairment	Dr. Guzzardi lacked practical, hands‑on training administering field sobriety tests and was therefore outside the scope to opine on their methodology or validation	Dr. Guzzardi’s scholarship, publications, lectures, review of validation studies, and prior testimony qualified him to opine that the tests are not validated to detect drug impairment	Court held exclusion was an abuse of discretion: Dr. Guzzardi was qualified to testify on the tests’ scientific validation and the exclusion prejudiced Taylor
Whether exclusion was harmless error	Excluded testimony would be cumulative or trivial given other evidence of impairment (pre‑accident driving and post‑accident behavior)	Exclusion deprived jury of key rebuttal to officer’s opinion diagnosing drug impairment based solely on sobriety tests	Court held error was not harmless beyond a reasonable doubt because the officer’s uncontradicted opinion was central to proving impairment; new trial required

Hyrcza v. West Penn Allegheny Health Sys., 978 A.2d 961 (Pa. Super. 2009) (standard of review for expert‑testimony admission and abuse of discretion)
Miller v. Brass Rail Tavern, Inc., 664 A.2d 525 (Pa. 1995) (liberal standard for qualifying expert witnesses)
Commonwealth v. Story, 383 A.2d 155 (Pa. 1978) (harmless‑error standard for reversal)
Commonwealth v. Williams, 573 A.2d 536 (Pa. 1990) (standards for when error is harmless)
Commonwealth v. Brennan, 696 A.2d 1201 (Pa. Super. 1997) (definition of reasonable probability that error contributed to verdict)
Commonwealth v. Sanchez, 82 A.3d 943 (Pa. 2013) (fact‑finder’s province to weigh witness credibility)