By William C . Thompson , Laurence D . Mueller , and Dan E .

experts sometimes disagree about the interpretation and statistical characterization of test results. This article will describe the key controversies and will explain what lawyers need to know to recognize and deal with controversial types of DNA evidence. When labs try to “type” samples that contain too little DNA, or DNA that is too degraded, the results of the DNA test can be unreliable. The test may fail to detect certain genetic characteristics (called alleles) of people who contributed DNA to the sample — a phenomenon called allelic drop out; the tests may falsely detect characteristics that did not come from contributors — a phenomenon called allelic drop in; and the test results may be distorted in other ways that complicate interpretation. Labs try to make allowances for these distortions when they deal with limited and degraded samples. Because two samples from the same person may (under these conditions) produce slightly different DNA profiles, labs must use lenient standards for declaring that a suspect “matches” or is “included as a possible contributor” to an evidentiary sample. They do not require that the suspect’s profile correspond exactly to the evidentiary profile — some differences are allowed in order to account for drop out, drop in, and other types of distortion. But this leniency in matching increases the probability that an innocent suspect, who was not a contributor, will be incriminated — and therefore requires an adjustment to the statistics that describe the value of the DNA match. The controversy we will examine concerns how big this adjustment should be, and how labs should make it. Before delving into that debate, however, we will provide some background information on DNA evidence.