U.S. flag

An official website of the United States government, Department of Justice.

Forensic DNA Education for Law Enforcement Decisionmakers

DNA Analysis, Interpretation, cont.

Home  |  Glossary  |  Resources  |  Help  |  Contact Us  |  Course Map

Time Required to Perform Laboratory Analysis

Step 3: DNA Analysis, Interpretation, cont.

For profiles or portions of profiles that pass quality thresholds, analysts may attempt to ascertain from whom the stain originated. This frequently begins with an assessment of the number of possible contributors to the stain. For profiles from one person (single source), direct comparisons are made to profiles from known individuals associated with the case. If the possible number of contributors is greater than one (a mixture of DNA profiles), a more intricate interpretation procedure is used. Typically, laboratories will attempt to determine if profiles from known individuals associated with the case could have contributed to portions of a mixed-stain profile. Interpretation of a DNA mixture can be very time-consuming. The details of this process are beyond the scope of this discussion.

If the interpretation confirms that a known sample in the case could have contributed to the evidentiary stain, a statistic will be presented to estimate the rarity of the association. In the United States, this is frequently expressed as a random match probability (RMP) or likelihood ratio (LR). An RMP is expressed as the rarity of the evidence profile in the general population (i.e. probability of randomly selecting an unrelated individual with this profile is one in x number of individuals). A LR is a comparison of two probabilities often referred to as the prosecution, or inclusion, hypothesis and the defense, or exclusion, hypothesis (i.e. given the DNA evidence, it is x times more likely if it originated from John Doe than if it originated from an unknown, unrelated individual). The larger the number, the more it favors the inclusion hypothesis.

The image below is a link to view a mixture profile of multiple donors.

Click to view mixture profile
National Institute of Justice (NIJ) (see reuse policy).

Back Forward