What is the test for establishing chain of custody?

People v. Riser (1956) 47 Cal.2d 566, 305 P.2d 1 sets forth the rules for establishing chain of custody: "The burden on the party offering the evidence is to show to the satisfaction of the trial court that, taking all the circumstances into account including the ease or difficulty with which the particular evidence could have been altered, it is reasonably certain that there was no alteration. [p] The requirement of reasonable certainty is not met when some vital link in the chain of possession is not accounted for, because then it is as likely as not that the evidence analyzed was not the evidence originally received. Left to such speculation the court must exclude the evidence. [Citations.] Conversely, when it is the barest speculation that there was tampering, it is proper to admit the evidence and let what doubt remains go to its weight." (Id. at pp. 580-581, 305 P.2d 1; see also People v. Williams (1989) 48 Cal.3d 1112, 1134, 259 Cal.Rptr. 473, 774 P.2d 146.)

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