Admissibility

Legal challenges to admissibility under the Federal Rules of Evidence in the US generally go under one of five categories. Evidence admitted has to be weighed by the trier of fact in making determinations. Depending on specifics of the circumstances and judicial opinion, evidence may or may not be admitted and weight may be expressed by the judge to the jury in formal admonitions for admitted evidence to go to weight.

Relevance: Relevance is defined as the tendency for evidence to make a fact of consequence determination of the action more or less probable than it would be without the evidence. In other words, evidence is relevant if and only if a consequential fact relating to the matter at hand can be reasonably interpreted by the trier of fact to be more likely to be true or less likely to be true by knowing the additional fact.

Authenticity: Rules 901-903 specify that in order for evidence to be admitted, there has to be evidence sufficient to support a finding that the matter in question is what its proponent claims. Many illustrative examples are provided, but they are not exhaustive. They include personal knowledge, non-experts familiar with a unique property such as handwriting, comparisons to known samples by the trier of fact or experts, distinctive characteristics, public records, ancient documents, reliable processes or systems, and methods provided for by statute or rule. Some records may be self-authenticating, such as public documents, certified copies of documents, official publications, and certified records of regularly conducted activity. In digital forensic evidence, these properties are not obviously present, and in isolation, digital forensic evidence is almost never able to be shown authentic through scientific methods.

Hearsay: Rule 801 specifies that an out of court statement offered in evidence to prove the truth of the matter asserted is hearsay, and thus not admissible. But there are many exceptions; most notably business records taken in the normal course of business and relied on for their accuracy and reliability as a matter of course in carrying out that business are typically admissible.

Original Writing: (also known as best evidence): Rules 1001-1008 indicate that to prove content, the original is required unless certain exceptions apply. Exceptions include: (1) original lost or destroyed, (2) original not obtainable, (3) the opponent who holds it refuses to produce it upon judicial demand, and (4) the content is not closely related to the matter at hand and is thus collateral. Official records are admitted as duplicates. Voluminous records may be represented by statistical samples when they are representative and subject to examination of the originals out of court. When the admission of other evidence depends on facts in this evidence, the court makes the determination, otherwise it goes to weight. When the issue is whether (a) the asserted content ever existed, (b) another piece of content admitted produced it, (c) the evidence in question accurately represents the original, the trier of fact determines it. For digital evidence, since exact copies are so easy to make, copies are commonly accepted, if they can be shown to accurately reflect the content of the original.

More probative than prejudicial: Rule 403 indicates that evidence may be excluded if its probative value is substantially outweighed by the danger of unfair prejudice, confusion of the issues, or misleading the jury, or by the considerations of undue delay, waste of time, or needless presentation of cumulative evidence. Since digital evidence often seems very convincing when it isn't necessarily as reliable as it may seem, it is quite prejudicial, but how probative it is depends on other factors. Precision in excess of accuracy, such as an estimate of a percentage to 6 or 7 digits of accuracy from a sample of only 100 items, is a potential problem because the evidence appears to be highly accurate, and thus should tend to be highly prejudicial, when it is not in fact as probative as it appears to be because the accuracy is not reflective of the real accuracy of the measurement.