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Comment by azalemeth | Hacker News Reader

I do often wonder about stories like this in the context of forensic science – my (incomplete!) understanding a lot of the time suspect DNA samples are taken from small areas and amplified significantly with high-cycle count PCR. I'd worry that any jury presented with a statistical argument about a fragment of somebody's DNA being very unlikely ("1 in 100 million") to be different to the sample found at the scene would not be aware of all of the potential systematic reasons why the actual true probability may be much, much higher.

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Probability seems to be one of those things humans habitually mess-up at.

"The chances of this person's unique DNA showing up at the scene are a zillion to one!"

"What does that really mean when the sample also contains unique DNA for a hundred other people? Did all of them commit the crime as a group?"

Depends on how they're using it. To find an unknown person and prove they were at a scene - yeah you'll have the 100 person's worth of DNA to sort through and then match against a (presently) incomplete DNA database. But if you already have a suspect and need to place them at the scene, if their DNA is one of the 100 then they have shown that.

But we’re they at the scene or did they just bump into someone or something that was there?

That's something that would have to be addressed at the trial by the defense attorney raising challenges.

If the DNA is present, it's present - barring any procedural mistakes by the forensics technicians (mislabeled sample, dirty lab equipment, didn't follow manufacturers instructions, etc). Or deceit by one or more members of the forensics team to implicate the suspect.

Except DNA may not be present: the probabilities involved with partial DNA matches become very problematic.

>That's something that would have to be addressed at the trial by the defense attorney raising challenges.

This is the wrong point at which to correct the problem. When prosecutors are allowed to introduce "science" as their evidence, jurors give this way too much deference. It's the prosecutor basically saying "you're too stupid to understand it, so indulge my appeal to authority" and jurors tend to happily acquiesce. This is why judges are supposed to gatekeep expert testimony and not just let any quack step in and make outrageous claims... and despite their attempts, quacks have repeatedly done just that these past few decades.

If is true that the presence of DNA means essentially nothing, then it shouldn't be permitted to be introduced at trial. Protection from wrongful conviction shouldn't rest on someone having an attorney expensive enough to be able to unsway too-easily-swayed jurors with counter-specious arguments.

Forensics never relies on a single piece of evidence. It takes several corroborating accounts and pieces of evidence to reach "beyond reasonable doubt."

This is why even breathalyzers have to be done twice, with a gap in between (and preferably with different sensors). It's also why several witnesses are examined. It's why fingerprints and lie detectors alone don't pass the muster. Nearly anything can be faked, or misinterpreted. All these things have to be used together to create an unbreakable story.

So there is next to no risk with DNA in the air.

As an example, there recently was a guy who went into a police station and claimed that he was relocating there from another province. The officer on duty was suspicious, and found there was no record of this officer in the entire country, and arrested him. In court, the guy got off the hook, even after impersonating a police officer and trying to infiltrate a station, because the officer on duty did not collect the appropriate evidence. The camera footage looked entirely normal. It became entirely hearsay.

You can pretty much expect him to charge that officer with wrongful arrest now.

We may never know what that guy was up to. But he certainly had chutzpah!

> Forensics never relies on a single piece of evidence

An uncharitable characterization is that forensics relies on many pieces of _stuff_ that cops and attorneys can reasonably convince a judge meets the rules of evidence and a jury that it is in fact evidence, while hoping the defense can't afford an expert (mind you, almost never a practitioner, but a professional legal consultant) to convince the jury it isn't evidence.

Most forensic analysis is complete bullshit, and it takes decades to convince judges to forbid the junk science.

So I wouldn't say there's "no risk." There's tons of risk.

What I would add is that you almost never have to worry about forensics if you're committing crimes, because the forensics are only going to be used to prove your guilt in court should you choose to fight it. If you're not committing crimes and become the focus of an investigation, you should be terrified of forensics.

This isn't unlike "the right to remain silent." It's weird that people don't fully understand that to mean stfu for your own good, 'cos the only notes they are writing are toward building their case against you.

Forensics isn't the way you characterized. There are forensic teams for both sides of every coin that needs forensics. It's literally a battle of what side can make the most compelling argument, and nothing more or less.

The most compelling argument is the one that has lots (the term "a preponderance" gets used in law where I am) of corroborating evidence through tools/techniques/people that are presumably not sharing and comparing notes or data with each other.

> it's literally a battle of what side can make the most compelling argument

So it's exactly as I characterized it, a racket. It's not science.

Almost all forensic tests are bullshit, and forensic experts simply cannot determine what they say they are able to under any scrutiny.

Samples are normally things like your semen being found in the dead body or your blood on the broken window, or the victim's blood on your shoes.

Air contamination is very unlikely. You aren't going to get air contamination inside the victim's bedroom... And if somehow a whiff of your air travelled a hundred miles into their bedroom and all over their corpse, other evidence would be used to rule you out.

Human DNA is relatively large. What's floating in the air is not going to be intact or complete.

Testing relies on amplifying many parts of the human DNA not just a small fragment.

The smaller the fragment the less precise the comparison.

So no, floating DNA does not meaningfully impact testing. It can impact PCR assays that target small regions for identification eg of viruses.

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