Generalization and Individualization

For decades, forensic science witnesses have testified that two samples, such as fingerprints, ballistics, or handwriting, are a “match” and that they are “100% certain” that no other person could have left the crime scene sample. The scientific reality is that no comparison of samples can result in 100% certainty. Using the term “match” implies, and is interpreted by jurors to mean, that there is no other person in the world who would have the same characteristics. Similarly, “certainty” claims are not scientifically based. Scientific conclusions, if possible, should be expressed in terms of probability or likelihood based on available data. Forensic science is becoming more reliant on quantitative analysis to validate evidence. Statistics are based on the collection and analysis of data, while probability is the likelihood that an event will occur based on data. The likelihood ratio is the ratio of two probabilities of the same event under different hypotheses. The best example is what has become the standard for testimony about DNA profiling, where the expert describes not a “match” but the likelihood of finding a person other than the defendant in the population with the same DNA characteristics as the crime scene DNA.