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Comment by baddox | Hacker News Reader

But surely you wouldn’t expect it to be perfectly accurate and repeatable. It’s pretty well known that no medical test I can think of is perfect.

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Pretty well known by who? The general population? I don't think so.

And there's a difference between "not perfect" and "doesn't provide the same result twice".

Yeah, I think most people would be surprised to learn that the same blood tests done by different labs can sometimes show 10% difference in values or even more. This could be the difference between a doctor declaring someone "within the normal range" or having a health problem.

"Not perfect" is the hand-waving way of excusing the imperfections and proceeding as though it were nearly perfect

"doesn't provide the same results twice" is the honest acknowledgement that the concept of perfection is itself what is flawed here because it creates false expectations of what science can give us in this arena

No no no, "science" can definitely deliver repeatable and accurate DNA sequencing, this much is beyond question. But that's expensive, and these companies sacrifice accuracy/repeatability to make it cheap.

The accuracy of these tests at these prices is a falsehood that science cannot support

The accuracy of the broad conclusions drawn from population wide samples about any single individual’s ethno-historical origins is a falsehood that science cannot ethically or logically support

"Not perfect" is when it fails a small number of times.

Not getting the same result twice is pretty much 100% failure :-)

I tend to agree, and since people are convicted and incarcerated on "DNA evidence," this seems an appalling and horrific indictment on the fairness of our court systems.

Comparing two DNA samples for identification of a culprit is totally different from determining the percentage of differing heritages that make up your DNA.

Frankly I am skeptical that they could even ascertain any semblance of your heritage but I'm definitely a layperson.

Really? Taking a pregnancy test more than once and getting a second opinion about a medical diagnosis are certainly both well established in public consciousness.

Maybe so but the kind of genotyping test that we are talking about is vastly more accurate than an over-the-counter pregnancy test, both in the mind of the public and in actual fact.

I had a one off blood pressure tests a few years ago that was outrageous 140 over something and completely unusual for me (which previously were like 110). The nurse was like, lets see what happens with a bigger cuff, and the reading from the automated machine was suddenly like 120 over something, and she was like, shrug you should watch it.

So I picked up a mid-range automatic bp cuff, and started to measure it more regularly. The numbers were all over the place and completely unrepeatable unless I maintained exactly the same position/breathing/etc. I can literally swing my BP by 30+ mmHg, by re-positioning my body, drinking a cup of coffee, etc, all while officially still maintaining proper technique for measuring it. And yes, moving up a cuff size also tends to remove a ~5mmHg from my measurement.

I could write a book about how to get any BP you want out of the machine. Its the same with weight, I can shift my weight +-10% by eating a bowl of salty popcorn drinking a few 64oz cups of ice tea, then eating a large meal. Then I can go for a 3 hour bike ride, use the toilet and drop it all back off.

So, its sort of surprises me the fine grained aspect of DR's always measuring your BP/Weight, neither of which really mean anything until they get into the extreem ranges, and in the weight case you can get a better feel for it just by looking at someone...

Where I trained, this is why we don’t diagnose someone as hypertensive unless there are three separate elevated blood pressure measurements on three different clinic visits.

We would schedule people for Nurse only BP checks to help establish the pattern.

This statistical anbiguity is why the medical community as a whole is often sceptical of the Direct to consumer DNA testing and SNP analysis. They report data in a convincing way which is statistically misleading and in no way plainly reveals what is within measurement error.

DNA tests are used in murder trials, I would expect them to be damn near perfectly repeatable.

Reading other replies there seems to be a cheaper less accurate tech used for these home testing kits.

The justice system deeply misunderstands both science and statistics.

It doesn't misunderstand them. It knows exactly what it's doing. It relies on judges and juries misunderstanding science and statistics in order to get results favorable to the prosecutors.

Basically all forensic science has turned out to be crap or misapplication of good science except for DNA which a) exonerates a heck of a lot of people and b) was a concept that matured in biology before being adopted by forensic scientists.

> DNA tests are used in murder trials, I would expect them to be damn near perfectly repeatable.

So are other forensic methods, like fingerprint[1] and hair[2] analysis. Turns out that forensic evidence isn't very scientific nor accurate.

[1] http://theconversation.com/fingerprinting-to-solve-crimes-no...

[2] https://rss.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1740-9...

Even if the tech was the same, couldn't they just run the less accurate test several times to increase the confidence?

That will reduce the random error, yes. But there's still the concern of systematic error, which has to be dealt with separately. If you're taking a biased sample to begin with, it doesn't matter how many times you repeat the test; the bias will still be there.

But wouldn't systematic error affect everyone's results in the same way, meaning it couldn't result in any particular pair of people becoming more or less likely to be matched as the same person?

DNA fingerprinting is so different from whole-genome characterization that it's hardly worth comparing them.

DNA fingerprinting involves:

* Sequence amplification of polymorphic sites by a panel of well-characterized primers, many of which cross-check each other by amplifying over the same locus.

* These amplicons are then characterized by size via restriction digest and simple electrophoresis. No sequencing is performed, only the length of the DNA fragments is characterized.

* Due to population-level variation of short-tandem repeats, analysis of ~20 loci can uniquely identify an individual with a desired level of statistical confidence. The kind of data used to confirm a match in e.g. a murder trial will be absurdly robust.

DNA fingerprinting is limited by the quality of sample collection and handling (i.e. don't mix up or contaminate samples), but not the accuracy of the method.

Your confusion is because you think there is one "DNA test". That isn't the case. You can sequence a genome, but that costs on the order of $1k so companies like 23andme don't offer that. They use simpler methods.

Even sequencing a genome is not perfect - the current most common method works by cutting the genome into pieces, sequencing those and then reassembling them. A pretty obvious limitation is that you can't detect repeated sections.

I'm not sure what method the police use but it definitely isn't the same as what 23andme do.

> DNA tests are used in murder trials, I would expect them to be damn near perfectly repeatable.,

So were the results of FBI hair and fiber analysis, and look how that turned out.

https://www.washingtonpost.com/local/crime/fbi-overstated-fo...

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