As I have noted in a previous edition of this column, acting as a juror in a criminal case can be a difficult proposition for the citizens selected. Prospective jurors must listen attentively to lawyers and expert witnesses speaking in unfamiliar nomenclature. At the conclusion of a lengthy trial, they retire to a windowless jury room to debate a weighty decision with perfect strangers, seeking unanimity. Under these circumstances, it is no surprise that a plethora of websites instruct people on how to be disqualified as a juror.

The brave souls who are willing to serve have often received their education about a criminal trial from television, a device which was apparently invented for the sole purpose of misrepresenting reality. Jurors often bring an amalgam of unrealistic expectations into the jury room, and unrealistic expectations can impact a criminal trial in a litany of concrete ways.

For example, in Virginia, most criminal forensic analyses are conducted by a state agency, the Department of Forensic Science. DFS maintains four regional labs which, like all state agencies, routinely face budget constraints. All northern Virginia police departments submit evidence to DFS’ Northern lab, meaning that facility is inundated with requests from Loudoun, Arlington and Fairfax in addition to Alexandria’s submissions. A lab that is overwhelmed by forensic requests must triage them by considering the gravity of the case and the likelihood of discovering inculpatory evidence.

While a television detective is never heard lamenting a lack of budgetary authority, in the real world a lack of time and money precludes the lab from conducting all potential forensic analyses in all cases. However, a wily defense attorney, whose sworn professional obligation is to get the best possible outcome for her client, can always conceive of another analysis that might have been conducted.

Perhaps the fact that budgets preclude detectives from conducting an infinite array of forensic examinations is unsatisfying, but it is the reality and a prudent prosecutor must explain that reality to jurors. In the same way, the prosecutor must disabuse jurors of the television-fueled presumption that incriminating forensic evidence, such as DNA and fingerprints, is routinely discovered at crime scenes.

Again, the fallacies of the television universe stand in stark comparison to the realities of criminal investigations. In the real world, a host of environmental factors like a suspect’s propensity to sweat, the temperature and the types of surfaces a suspect may have touched conspire with pure dumb luck to make it unlikely that any forensic evidence left behind by the suspect will be discovered. I have personally conducted the direct examinations of expert witnesses who explained that relevant DNA or fingerprints is recovered in approximately 10 percent of cases.

Obviously, then, 90 percent of all forensic examinations attempted by police result in failure. This immutable fact of real-world criminal investigations gives our wily defense attorney another credible argument to present to a credulous jury, because every examination that fails to inculpate a defendant is an examination that may seem to exonerate them in light of the subconscious bias of jurors.

In effect, the defense attorney is saying to the jurors: “You know from television that incriminating forensic evidence is easy to obtain, but none was recovered in this case. Does that mean the police are incompetent? Or perhaps that my client is innocent? Either way, you can’t convict.”

A seasoned prosecutor attacks the jury’s unrealistic preconceptions head-on. Beginning with jury selection, the prosecutor explains the differences between the gossamer fantasies portrayed on T.V. and the hard realities of the real world. Jurors are selected who agree to try the case based on the evidence adduced at trial and not on their recollection of plot lines from “CSI:Miami.” During direct examination, the prosecutor allows expert scientific witnesses to explain to the jurors how difficult it is to recover and identify incriminating forensic evidence. Finally, the prosecutor explains that a failure to isolate the defendant’s DNA or prints does not necessarily indicate innocence.

As I often tell a jury, the absence of evidence is not evidence of absence.

The writer is the commonwealth’s attorney for Alexandria.