Forensic expert witnesses carry unusual authority in criminal trials. A jury that might skeptically evaluate an eyewitness’s account often defers to a laboratory analyst who presents technical findings in confident, technical language. The perception that science is objective (that a GC-MS result or a DNA match speaks for itself) can overwhelm the jury’s usual critical faculties.

The effective challenge to a forensic expert does not argue that science is unreliable in general. It often goes in one of two directions: 1. This analyst, performing this test, under these specific conditions, on this sample, produced a result whose reliability has not been adequately demonstrated or 2. This analyst, performing this test, did not follow proper protocols and procedures. These arguments require understanding both the science and the specific record, which is why forensic science training matters in ways that legal training alone does not address.

The Two-Stage Challenge

Forensic expert testimony can be challenged at two distinct stages in a Texas criminal case. The first is a pretrial admissibility challenge under the Texas equivalent of the Daubert standard. The second is cross-examination at trial.

Pretrial: The Daubert/Kelly Admissibility Challenge

Texas Rule of Evidence 702 governs the admissibility of expert testimony. In E.I. du Pont de Nemours and Co. v. Robinson (1995), the Texas Supreme Court adopted a framework similar to the federal Daubert standard, often referred to in Texas practice as the Robinson framework or, for criminal cases, the Kelly standard (from Kelly v. State, 824 S.W.2d 568 (Tex. Crim. App. 1992)). Under this framework, the trial court acts as gatekeeper and must assess whether proposed expert testimony is based on a sufficiently reliable methodology before it can be presented to the jury.

The factors the court considers include: whether the underlying theory can be and has been tested; whether the theory has been subjected to peer review and publication; whether the technique has a known or potential rate of error; whether standards exist and are maintained for operation of the technique; and whether the theory or technique has been generally accepted in the relevant scientific community.

A Daubert/Kelly hearing requires filing a motion challenging the admissibility of the specific forensic evidence and requesting an evidentiary hearing at which the prosecution must establish the reliability of the proposed testimony. At the hearing, the court can hear testimony from forensic experts on both sides and review the scientific literature.

This is where the NAS Report and the PCAST Report become litigation tools. In any case involving bite mark analysis, microscopic hair comparison, firearms identification, or footwear analysis, the defense can present these reports (authored by leading scientists and specifically concluding that the disciplines lack foundational validity) as scientific authority for the proposition that the evidence does not meet the reliability threshold for admission.

Trial: Cross-Examination of the Forensic Analyst

Cross-examination of a forensic analyst requires knowing what questions to ask, which requires knowing what the answers might reveal. A cross-examination built on general attacks on forensic science (“isn’t it true that labs make mistakes?”) rarely accomplishes anything. A cross-examination built on the specific record (the analyst’s training file, the calibration logs, the method validation documentation, the proficiency test results, the chain of custody records) can expose the specific weaknesses in this result, in this case.

The productive areas for cross-examination of a forensic toxicologist or drug chemist include:

  • Training and qualifications.  Has the analyst completed all required training? Are they currently certified by a recognized professional body? What proficiency tests have they taken and what were the results?
  • Method validation.  Has the laboratory validated the method used in this case for this analyte in this matrix? Can the analyst produce the validation documentation? Does the laboratory’s method match the scientific literature for this type of analysis?
  • Calibration and instrument performance.  Was the instrument properly calibrated before this analysis? Did all calibration standards fall within acceptance criteria? Were there any instrument alerts or error conditions during the run?
  • Chain of custody.  Who collected the sample? How was it labeled? How was it stored? How was it transferred to the laboratory? Who signed the chain of custody at each step? Were there any breaks in the documented chain?
  • The analyst’s personal involvement.  Did the analyst personally perform the analysis, or did they review data generated by someone else? Under Bullcoming, a surrogate analyst cannot adequately substitute for the certifying analyst.
  • Uncertainty and error.  All quantitative measurements have uncertainty. What is the expanded uncertainty of the reported value at the 95 percent confidence level? Does the laboratory report this uncertainty? If the true value is at the lower end of the uncertainty range, does it fall below a relevant threshold?

The Role of Defense Experts

In complex forensic science cases, retaining an independent defense expert is often essential. A defense expert can review the laboratory’s raw data and methods, identify errors and methodology issues the defense attorney might recognize but not be equipped to articulate at trial, and testify to alternative interpretations of the evidence.

The most valuable defense experts are those who work in the relevant field (ex. forensic toxicologists for drug and alcohol cases, forensic DNA analysts for DNA evidence cases) and who can speak to the scientific standards of the discipline with authority. An attorney who can evaluate an expert’s qualifications and their analysis at the forensic science level is better positioned to select the right expert and to effectively present their testimony.

 

If forensic science evidence is central to your case, contact Deandra Grant Law for a free, confidential consultation. Managing Partner Deandra Grant and Partner Douglas Huff both hold the ACS-CHAL Forensic Lawyer-Scientist designation and Deandra has a Master’s Degree in Pharmaceutical Science and a Graduate Certificate in Forensic Toxicology. Call (214) 225-7117 or visit deandragrantlaw.com.