Probability, DNA and court

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I recently initiated the Wikipedia article about the Court here in Lyngby and surfed a bit round on the homepage of the court. I stumbled upon an interesting case from 2011 about acquittal with a DNA match.

Here the case is with my translation and editing:

A 31 year old man from the western suburbs of Copenhagen was acquitted from accusations of theft totalling 200,000 Danish kroner from a villa in Gentofte. The evidence against the accused was alone a DNA match which meant that there were more than 1,000,000 times larger
probability for, that blood found on the site came from the accused than from another random person in the Danish population.

The accused which was first interrogated ca. 4 months after the crime, was not previously convicted for theft, had a well-paid job, own house, girlfriend and 2 kids and refused any knowledge of the theft.

The police did not perform any other investigations and the court found that the accused lived approximately 11.7 kilometers from the site of theft – like approximately 1,000,000 other person.

The court noted that the police could have performed a search at the house of the accused, could have obtained mobil phone records and looked for scars on the accused.

This is indeed a very interesting case of probabilities. A probability of 1 against 1,000,000 is probably not right. Unfortunate errors such as contaminations or paper work errors change the probability perhaps to as low a 1 against 1,000. When such errors are taken into account
they work in favor of the accused.

One may ask how they obtained the accused and his DNA in the first place. Was he suspected? Prior non-theft convictions? If he was suspected prior to the DNA test then even 1 against 1,000 is serious odds. It is much less serious if the man was routinely entered into a DNA register and the match appeared after a search in the database containing perhaps several 100,000 people.

It would be really interesting if the police appealed the decision and made more through investigations, e.g., are they able to track the mobil phone of the accused. It is important to know how such big odds should be interpreted and trusted.


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