- In 1969, Jane Mixer, a law student, was murdered. The case went cold.
- The case was reopened 33 years later when crime-scene evidence was submitted to DNA analysis.
- The DNA yielded two matches; both matches were from samples that were analyzed in the same lab and at the same time of the crime-scene DNA analysis. All three samples were analyzed in late 2001 and early 2002.
- One match was to John Ruelas. Mr. Ruelas was 4 in 1969 and was excluded as a suspect.
- The other match was to Gary Leiterman. Mr. Leiterman was 26 at the time. He was convicted in 2005 and is serving life without parole. His appeal was denied in 2007.
- There is no doubt that Mr. Leiterman's DNA was deposited on the crime scene sample. The match is 176-trillion-to-1.
- The question is whether the DNA was deposited at the crime scene in 1969 or if there was a cross-contamination event in the lab in 2002.
A Very Easy and Helpful PowerPoint:
- This case comes from John Wixted, a psychologist at UCSD
- He has made a detailed and convincing presentation. Click here for The Power Point from John's website.
- John has helped to persuade the Innocence Clinic at the University of Michigan to investigate the Leiterman case.
- John and I are convinced this is a an injustice. We are working pro bono.
- Our job is to make an educated assessment of Mr. Leiterman's guilt or innocence. It would greatly help the Innocence Clinic to assess whether there is sufficient evidence to appeal.
- The jury heard that DNA is a trillion-to-1 accurate and there was only a very tine chance of cross contamination. Yet, we know these are the wrong conditional probabilities to compute.
- Consider the two hypotheses above that Leiterman's DNA was deposited at the crime scene or, alternatively, that it was deposited in the lab through cross contamination. Conditional on the match, compute posterior probabilities.
I have done my own analyses and typeset them. But reasoning is tricky, and I would like some backup. It is just too important to mess up. Can you try your own analysis? Then we can decide what is best.
You will need more information. I used the following specifications. Write me if you want more:
- John and I assumed 2.5M people are possible suspects in 1969. It is a good guess based on population estimate of Detroit metro area.
- The lab processes 12,000 samples a year. The time period the DNA overlapped can be assumed to be 6 months, that is 6,000 other samples could be cross contaminated with Mixer or Leiterman.
- The known rate of DNA cross-contamination is 1-in-1500. That is, each time they do a mouth swab from one person, they end up with two or more DNA profiles with probability of 1/1500. We assume this rate holds for unknowable cross-contamination such as that in processing a crime scene.
- The probability of getting usable DNA from a 33-year-old sample is 1/2.
- Need other facts? Just ask in the comments.
Jeff's answer is at GitHub, https://github.com/rouderj/leiterman