What is the legal test for establishing that a police identification procedure is suggestive or improper?

It is well settled that a suggestive or otherwise improper identification procedure violates due process and is not admissible to determine the guilt or innocence of a defendant (U.S. v. Wade, 388 US 218). At a Wade hearing, the People are required to establish the reasonableness of the police conduct and the lack of any undue suggestiveness in regard to the pre-trial identification procedures employed (see, People v. Chipp, 75 NY2d 327 [1990]). The burden then shifts to the defendant who must establish, by a preponderance of the evidence, that the identification procedure employed with regard to him was impermissibly suggestive and conducive to an irreparably mistaken identification (Id.). With regard to the instant hearing, the People met their burden. Defendants did not.