Phrases for the Value or Weight of Evidence

A few statisticians asked me (independently) about the usage of the terms evidential value, evidentiary value, and probative value. For years, I thought the phrases all meant the same thing, but that is not true in some fields.

Evidential Value

Black’s Law Dictionary (which tends to have aged definitions) has this definition of evidential value: “Value of records given as or in support of evidence, based on the certainty of the records origins. The value here is not in the record content. This certainty is essential for authentic and adequate evidence of an entity’s actions, functioning, policies, and/or structure.”

Under this definition, "evidential value" pertains to a document's value merely as the container of information. The definition distinguishes between the provenance and authenticity of a document -- where did it come from and has it been altered? -- and the content of the document -- what statements or information does it contain? Likewise, archivists distinguish between "evidential value" and "informational value." The former, according to the Society of American Archivists "relates to the process of creation rather than the content (informational value) of the records."

Evidentiary Value

Lawyers use the phrases "evidentiary value" and "probative value" (or "probative force") as synonyms. For example, a 1932 note in the University of Pennsylvania Law Review on "Evidentiary Value of Finger-Prints" predicted that "the time is not far distant when courts must scrutinize and properly evaluate the probative force to be given to evidence that finger-prints found on the scene correspond with those of the accused." \1/

Forensic scientists use "evidentiary value" to denote the utility of examining objects for information on whether the objects have a common origin. A 2009 report of a committee of the National Academies complained that there was no standard threshold for deciding when bitemarks have "reached a threshold of evidentiary value." \2/ More generally, the phrase can denote the value of any expert analysis of effects as proof of the possible cause of those effects. \3/

Evidential Value

Unlike archivists, forensic scientists use the phrase “evidential value” interchangeably with "evidentiary value." It appears routinely in titles or articles and books such as "Evidential Value of Multivariate Physicochemical Data," \4/ "Enhancing the Evidential Value of Fingermarks Through Successful DNA Typing," \5/ and "Establishing the Evidential Value of a Small Quantity of Material Found at a Crime Scene." \6/

Probative Value

Lawyers use "probative value" to denote the degree to which an item of evidence proves the proposition it is offered to prove. Credible evidence that a defendant threatened to kill the deceased, whose death was caused by a poison, is probative of whether the death was accidental and whether defendant was the killer. With circumstantial evidence like this, various probability-based formulations have been proposed to express probative value quantitatively. \7/ One of the simplest is the likelihood ratio or Bayes factor (BF) favored by most forensic statisticians. \8/ Its logarithm has qualities that argue for using log(BF) to express the "weight" of an item of evidence. \9/

The rules of evidence require judges to exclude evidence when unfair prejudice, distraction, and undue consumption of time in presenting the evidence substantially outweigh the probative value of the evidence. \10/ In theory, judges do not exclude evidence just because they do not believe that the witness is telling the truth. The jury will take credibility into account in deciding the case. However, in ensuring that there is sufficient probative value to bother with the evidence, judges can hardly avoid being influenced by the trustworthiness of the source of the information. Moreover, the importance of the fact that the proposed testimony addresses and the availability of alternative, less prejudicial proof also can influence the decision to exclude evidence that is probative of a material fact. \11/

NOTES

1. Note, Evidentiary Value of Finger-Prints, 80 U. Penn. L. Rev. 887 (1932).
2. Comm. on Identifying the Needs of the Forensic Sci. Cmty. Nat'l Research Council, Strengthening Forensic Science in the United States: A Path Forward 176 (2009).
3. Nicholas Dempsey & Soren Blau, Evaluating the Evidentiary Value of the Analysis of Skeletal Trauma in Forensic Research: A Review of Research and Practice, 307 Forensic Sci. Int'l (2020), https://doi.org/10.1016/j.forsciint.2020.110140. Still another usage of the term occurs in epistemology. See P. Gärdenfors, B. Hansson, N-E. Sahlin, Evidentiary Value: Philosophical, Judicial and Psychological Aspects of a Theory (1983); Dennis V. Lindley, Review, 35(3) Brit. J. Phil. Sci. 293-296 (1984) (criticizing this theory).
4. Grzegorz Zadora, Agnieszka Martyna, Daniel Ramos & Colin Aitken, Statistical Analysis in Forensic Science: Evidential Value of Multivariate Physicochemical Data (2014).
5. Zuhaib Subhani, Barbara Daniel & Nunzianda Frascione, DNA Profiles from Fingerprint Lifts—Enhancing the Evidential Value of Fingermarks Through Successful DNA Typing, 64(1) J. Forensic Sci. 201–06 (2019), https://doi.org/10.1111/1556-4029.13830.
6. I.W. Evett, Establishing the Evidential Value of a Small Quantity of Material Found at a Crime Scene, 33(2) J. Forensic Sci. Soc’y 83-86 (1993).
7. 1 McCormick on Evidence § 185 (R. Mosteller ed., 8th ed. 2020); David H. Kaye, Review-essay, Digging into the Foundations of Evidence Law, 116 Mich. L. Rev. 915-34 (2017), http://ssrn.com/abstract=2903618.
8. See Anuradha Akmeemana, Peter Weis, Ruthmara Corzo, Daniel Ramos, Peter Zoon, Tatiana Trejos, Troy Ernst, Chip Pollock, Ela Bakowska, Cedric Neumann & Jose Almirall, Interpretation of Chemical Data from Glass Analysis for Forensic Purposes, J. Chemometrics (2020), DOI:10.1002/cem.3267.
9. Fed. R. Evid. 403; Unif. R. Evid. 403; 1 McCormick, supra note 6, § 185.
10. I. J. Good, Weight of Evidence: A Brief Survey, in 2 Bayesian Statistics 249–270 (Bernardo, J. M., M. H. DeGroot, D. V. Lindley & A. F. M. Smith eds., 1985); Irving John Good, Weight of Evidence and the Bayesian Likelihood Ratio, in The Use of Statistics in Forensic Science 85–106 (C. G. G. Aitken & David A. Stoney eds., 1991).
11. 1 McCormick, supra note 6, § 185.