Confidence Interval of the Likelihood Ratio Associated with Mixed Stain DNA Evidence


  • Supported in part by NIH grants ES 07329, GM 45344 and GM 075091.

Additional information and reprint requests:
Bruce S. Weir, Ph.D.
University of Washington
Department of Biostatistics
Box 357232
1705 NE Pacific Street
Seattle, WA 98198-7232


Abstract:  Likelihood ratios are necessary to properly interpret mixed stain DNA evidence. They can flexibly consider alternate hypotheses and can account for population substructure. The likelihood ratio should be seen as an estimate and not a fixed value, because the calculations are functions of allelic frequency estimates that were estimated from a small portion of the population. Current methods do not account for uncertainty in the likelihood ratio estimates and are therefore an incomplete picture of the strength of the evidence. We propose the use of a confidence interval to report the consequent variation of likelihood ratios. The confidence interval is calculated using the standard forensic likelihood ratio formulae and a variance estimate derived using the Taylor expansion. The formula is explained, and a computer program has been made available. Numeric work shows that the evidential strength of DNA profiles decreases as the variation among populations increases.