The Kinship Problem
Nationwide, forensic DNA labs have quietly been encountering troubling results when testing the newest computerized DNA analysis methods. Complex DNA mixtures give analysts real difficulty. To help with this, law enforcement recently began replacing human analysts with a computer program for some portions of that analysis. The software—generically called probabilistic genotyping—claims to reduce analytical error. Some labs are discovering, however, that the software can get a critical portion of cases wrong—badly wrong.
Take the Los Angeles County Sheriff’s Department’s Scientific Services Bureau. In their internal validation study of a probabilistic genotyping system, the Bureau created experiments testing DNA mixtures made up of first-order relatives. First-order relatives include an individual's parents, full siblings, and children. They did this specifically to figure out “how well the software can distinguish between closely-related individuals.” The results were abysmal.
Almost 90% of tested mixtures made up of 3 people or more produced results that falsely included a person the testers knew was not included in the mixture. In other words, the false positive error rate was 87%.
And those erroneous conclusions were not insignificant. Even in these false positive cases, the system claimed high confidence in its result. The Bureau warned: “Analysts should use caution when interpreting mixtures believed to be comprised of first-order relatives.”
And LA County is not alone. We’ve found eight other labs that encountered the same thing. On a scale where any value greater than 1 is inclusionary and larger magnitudes indicate elevating confidence levels, all of these labs discovered troubling false positives when relatives are involved:
|DNA Crime Lab||Highest Non-Related||Highest Related||Difference as a factor of|
|LA County Sherriff's Dept.||101||1015||100,000,000,000,000|
|Sacramento Cty. D.A.'s Crime Lab||101||1013||1,000,000,000,000|
|Palm Beach Cty. Sheriff's Office||106*||1017||100,000,000,000|
|Las Vegas Metropolitan Police Dept.||103||1013||10,000,000,000|
|Colorado Bureau of Investigation||102||1010||100,000,000|
|Jefferson Cty. Regional Crime Lab||102||109||10,000,000|
|DNA Labs International||102||107||100,000|
|Wisconsin State Crime Lab||102||107||100,000|
|Oregon State Police Portland Metro||104||106||100|
The nine lab’s internal validation summaries discussed above are available NOW at the links below.
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- ••• Palm Beach County Sheriff’s Office (PBSO) Laboratory - Internal Validation of STRmix v. 2.4 (FusionTM 5C)
- ••• Los Angeles County Sheriff's Department, Scientific Services Bureau Biology Section - Validation of STRmixTM v. 2.5.11 using the Powerplex Fusion 6C Kit
- ••• Jefferson County Regional Crime Laboratory - Internal Validation of STRmix™ v. 2.6 for the Analysis of GlobalFiler™ Profiles
- ••• Sacramento County District Attorney’s Crime Laboratory -Internal Validation of STRmix™ v. 2.4
- ••• Las Vegas Metropolitan Police Department - Internal Validation of STRmix™ v2.6
- ••• Colorado Bureau of Investigation - Internal Validation of STRmix™ v. 2.5 for the CBI Forensic Laboratories
- ••• DNA Labs International - From Training to Trial: A Reflection on Nearly Three Years of Probabilistic Genotyping
- ••• DNA Labs International - Probabilistic Genotyping in the Courtroom – Admissibility, Families, Secondary Transfer and Competing Statistics
- ••• California Department of Justice —Casework Internal Validation Summaries of STRmix v 2.0.6 for the Bureau of Forensic Services (Published by Electronic Privacy Information Center)
- ••• DNA Labs International - It Runs in the Family – or Does it? – Using Probabilistic Genotyping to Evaluate Possible Familial Relationships in Complex Mixtures
- ••• Wisconsin State Crime Laboratory - Internal Validation Summary for STRmix™ Probabilistic Genotyping Software
- ••• Oregon State Police, Forensic Services Division, Portland Metro Laboratory - Validation Study for STR Analysis Volume 67—2016 Validation – STR Casework Analysis using GlobalFiler, the 3500xl, and STRmix