In Arizona, as in any other state, the police and prosecution have the burden to prove the defendant guilty on reasonable doubt. Criminal investigators and prosecutors use a variety of different kinds of evidence in order to do that. The strongest type of evidence is physical evidence. Physical evidence comes in variety of types. Physical evidence can be objects used in a crime, e.g., guns, knives, automobiles, etc. Physical evidence might consist of an object that somehow corroborates the charge (or exonerates the accused), like a green shirt that was worn at the time of the offense. Physical evidence can also be analyzed to lead to scientific opinion evidence. This category would include fingerprints, DNA analysis, blood analysis, other trace and chemical types of evidence. Whatever kind of physical evidence might exist, it carries weight because it is objective.
Eyewitness testimony, on the other hand, is very subjective and is fraught with problems. Eyewitnesses can have a bias or motive to testify a certain way. Or, they can simply lack the necessary perspective to accurately portray what happened during the course of an alleged crime. Eyewitness testimony is subject to challenge and is far inferior, from the investigators' and prosecutors' perspective, to physical evidence. And the worst eyewitness testimony of all, the weakest of the weak, is eyewitness identification testimony.
Eyewitness identification testimony has been proven again and again to be unreliable. The testimony has been unreliable for a variety of reasons and in a variety of ways. In cases of later exoneration through DNA evidence, faulty eyewitness identification has been seen to play a major role in the wrongful initial conviction. Perhaps the most prevalent instance of faulty identification comes in any circumstance where the identification is made across race or ethnicity.
Unfortunately, despite the inherent problems with this subjective eyewitness identification testimony, juries often put much more weight on eyewitness identification than is justified by the real world facts. For this reason, a recent Arizona Court of Appeals is good news for the cause of justice. In State v. Nottingham, the Court of Appeals set out a rule for all cases in which eyewitness identification is a major part of the prosecution's case – the rule requires the trial courts to give a jury instruction warning the jury to take special care in appraising any eyewitness identification evidence.