Forensic scientists are tasked with the job of explaining often complex scientific data to judges, jurors, and attorneys who may have little understanding of the science underlying the forensic analysis in a case.
Revelations in recent years that hair analysts provided testimony that had no scientific validity has highlighted the importance of using correct language to convey information about forensic analysis and its limitations. Several wrongful convictions have been overturned based on faulty testimony about hair evidence and a massive review of hair cases is ongoing.
In an effort to improve expert testimony, the U.S. Department of Justice has released proposed guidelines called the Uniform Language for Testimony and Reports. These guidelines, once finalized, will apply to all USDOJ personnel who issue reports or provide expert forensic testimony, and they will probably be viewed as best practices for the entire forensic community. The guidelines are currently accessible at https://justice.gov/dag/forensic-science, with the public comment section running through July 8, 2016.
Attorneys should be aware of these guidelines and be prepared to object to any lab report language or testimony that does not comply. For example, the Uniform Language for Testimony and Reports for the Forensic Latent Print Discipline provides the following guidance on what experts cannot say about their evidence: (1) they cannot state that two prints originate from the same source to the absolute exclusion of all other sources; (2) they cannot state that there is any statistical level of certainty attributed to their determination; (3) they cannot testify that their science is infallible. Similarly, for the testing of bodily fluids, analysts may not state or imply that a level of numerical certainty is calculated to support the identification of blood or semen (i.e., they cannot say they are 95 per cent confident that the stain contained blood or there is a one in a hundred chance that the stain was something other than semen) and may not state or imply that the methods used in performing serological examinations have error rates of zero or that they are infallible. For drug analysis, the guidelines specify that when no sampling plan was used and no reasonable assumption of homogeneity of an item was determined, the examiner may not report or state an opinion that the conclusions apply to the entirety of an item (or a percentage of the item). This limitation is important in pill cases where only one pill is tested.
So, if you have any suggestions about appropriate limitations for expert testimony, or the language they should use in reports, please take the time to provide feedback on these documents.