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The likelihood ratio (LR) is the recommended approach for forensic DNA mixture analysis by the DNA commission of the ISFG, as it makes maximum use of available data and parameters for allelic drop-out and drop-in can be incorporated. We have developed and validated a LR method and software for analysis of mixed evidence samples in which drop-out of true contributors′ alleles and drop-in of extraneous alleles may have occurred. This method, the forensic statistical tool (FST), employs empirically determined drop-out and drop-in probabilities for single source samples and mixtures. The LR is computed for pairs of prosecution and defense hypotheses based on sample characteristics specified by the user. Data from up to three evidence amplifications may be considered simultaneously. The performance of the program was evaluated with hundreds of profiles generated from blood and buccal samples, purposefully degraded samples, and touched items with one, two, three, and four known contributors. The validation demonstrated that FST assigns an appropriate weight to all types of comparisons. FST is now being used in casework and results continue to be consistent with qualitative assessments. |
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