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How experts can sometimes hurt a criminal case

Forensic evidence, including fingerprints and ballistics analysis, is prominently used in many criminal prosecutions in New Mexico and around the country. While fingerprint and ballistics analysts are commonly allowed to testify as experts in criminal cases with few challenges, research has demonstrated that their assertions are not infallible and might be inaccurate. This is a real problem for people who are charged with crimes based on these types of evidence and little else.

Fingerprint evidence not always accurate

While fingerprint evidence is generally considered to be unassailable, research has shown that experts rely on the misleading use of math to testify to its degree of conclusiveness. For example, fingerprint analysts look at sets of fingerprints and state whether they are a match, nonmatch, or inconclusive. Inconclusive determinations are not considered to be incorrect but are disregarded. In a test of fingerprint analysts, they were given the same sets of fingerprints seven months apart. When the analysts considered the same sets of fingerprints the second time, 10% reached different conclusions than they had originally. When analysts testify, however, they testify that their conclusions have a high degree of accuracy, which might not be true.

Problems with ballistics evidence

Ballistics evidence is also problematic. Experts might testify with a high degree of confidence that a particular shell casing came from the gun found in a defendant’s possession. Ballistics experts use the same match, non-match, or inconclusive system and disregard results that are inconclusive. One study found that 98.3% of the findings were inconclusive. However, since the experts disregard all of the inconclusive results, they might testify that their conclusions have an extremely high accuracy rate when they do not.

Forensic evidence is problematic because it might not have a high degree of reliability or validity, but judges commonly allow it into evidence. Juries can be swayed by this type of evidence, making it important for criminal defense attorneys to understand the science underlying it and effectively challenge its admissibility.