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Examining the Evidence

DNA Testing

Forensic DNA analysis has played a crucial role in the investigation and resolution of thousands of crimes since the late 1980s.

Experience And Judgment

The current understanding of DNA was developed through decades of rigorous scientific study and testing.

Though the introduction of DNA analysis into the courtroom was vigorously challenged and debated, DNA evidence is now treated as the gold standard of modern forensic techniques. The National Academy of Sciences report went so far as to suggest that other forensic techniques should strive for a similar caliber of reliability.

Even with such a strong scientific foundation, successful DNA testing relies on the skills of experienced analysts. The accuracy of the test depends on the attentiveness and experience of those examiners, as does the evaluation of the results. Analysts often need to make judgment calls in assessing outcomes, particularly with specimens involving multiple source samples. Also, a depth of experience is called for in efficiently identifying and troubleshooting problems.

Random Match Probability

When the DNA profile of a specimen in evidence matches that of a known specimen, analysts need to know how rare that match is. In other words, how many other unrelated people in the population would have the same profile? To establish this likelihood, examiners estimate the “random match probability” or RMP.

Calculating RMP might result in a very small number, on the order of a 1-in-a billion chance for a match, or even less. It’s a good probability estimate, but it’s important to remember that this is not a count. If you tested the DNA type of a billion people, you would not necessarily find one, and only one other profile to match.

Profile Rarity

Another very important consideration in calculating RMP is ethnic variation. Although race or ethnicity is much more a social construct than a genetic one, it is known that genotype frequencies vary among identifiable ethnic groups (e.g. Caucasians, African-Americans, and so on). An accurate estimation of the rarity of a profile will take these distinctions into account. The lab must have a database of samples from the appropriate ethnic group from which to get its estimate of the genotype frequencies, in order to do the multiplication. Depending on the ethnic group and genotypes, the numbers can vary considerably.

Over the years, a vast number of profiles have been collected. In fact, the expansive database of available profiles for use in comparison is a principle reason for the robust nature of DNA testing. No other forensic discipline has acquired this depth of population data.

One more important factor that analysts take into consideration when calculating RMP is that the estimate assumes the profiles in question are from unrelated people. If relatives might be depositors of the evidence, the calculations need to change, and the probability of a random match becomes markedly higher.

Learn how the use of statistics and probability lead to some common DNA fallacies as evidence is weighed in the courtroom.