Within two weeks I have learnt that bite mark and hair analysis are full of BS. Is there some list of all these shoddy methods?
Every new kind of forensics is treated like it's going to be the solution to solving crime, when what it really is, is yet another excuse not to put the time and effort into investigation, the one thing that actually solves crimes.
CCTV has the same problem, we think it will solve crimes, when it can actually do a lot, but we constantly see cases of crimes happening in public or on camera where the police haven't even taken the half hour it would take to ask local businesses for their footage. And when they even do _that_, then they don't rarely bother to follow up leads.
Policing is just an endless, endless stream of people with what are actually very important jobs who only go into it for exactly the wrong reasons.
The problem is that there are some attempts to make weaker test and try to confuse people so they think it's the real test. For example: https://en.wikipedia.org/wiki/DNA_profiling#Partial_matches
Quoting the Wikipedia article:
> We can see no reason why partial profile DNA evidence should not be admissible provided that the jury are made aware of its inherent limitations and are given a sufficient explanation to enable them to evaluate it.
That sound good, but every time I read it again it looks more and more fishy.
Say you can confirm a sample is 100% a certain person. How do investigators have any idea how it got there? How do you know it’s from a criminal and not the guy that stocked the convenience store shelves?
I tried to look into this and maybe I was searching the wrong thing but found nothing.
They often can't be certain, and I'm sure somebody is in prison because they're DNA was in an inconvenient place, but it's a far superior form of evidence than the other methods mentioned.
Unless a crime is widely watched in person with supporting recordings and DNA evidence, it's basically impossible to be a 100% certain of what happened. That's why court only attempts to prove guilt beyond reasonable doubt.
Keeping a massive DNA database and using it to convict someone, anyone because a crime occurred near something they touched before would be fallacious and unjust. However, if we already had other reasons to suspect someone, and DNA gives us another, that's much more powerful.
This is the same way we guarantee the integrity of elections.
There are two kinds of witness testimony - the most common is where a witness sees a stranger and testifies about it. Those have little credibility.
But there's another kind, where the witness actually knows the person, those are far more credible. Historically only this type was permitted, but over time things seem to have shifted to the first type.
I don't know how far forensic science has come since a decade ago when I learned some of it, but at that point they were running "feature matching" instead of full-fingerprint matching. In other words, they looked for aberrancies (e.g. "islands," "deltas," "bifurcations" etc. at specific spots), and then checked those against a finger-to-person database. Then a list of possible matches would be found, if they also contained some set amount of same features in the same places.
There are two main issues here.
First, while taking an optical scan can provide very high-resolution images of a fingerprint -- field data is never like that. Fingerprints that are collected usually are not uniform, and dependent on pressure applied by the finger(s) in question and the amount of oil left behind (which powder sticks to "reveal"). Because of this, you're never going to get a perfect fingerprint in the field. Some areas will be faint, misleadingly separated (and even have false artificial aberrancies not present on the original finger), or outright missing -- leading to less "features" to check against. Others will be completely "blotched" out by a high amount of oil, removing vital information.
Second, you will almost never get an exact match because of this -- you will be getting a list of possible matches. I can remember one notable case of a terrorist's prints being wrongly attributed to someone who could never have been at the scene, much less anywhere near the country -- but was still whisked away by (I believe?) Interpol. Forensic evidence is just a piece of numerous possible stories that can be spun up any which way. There's something to be said here about human biases, incentives, and so on -- but I think it's intuitive enough (and frankly, I don't want to write more).
https://www.loevy.com/blog/the-fallout-continues-from-massac...
"Last month, the Massachusetts Office of the Attorney General began notifying approximately 7,500 people that their drug convictions have been overturned."
There's a graphic about halfway down that shows supposed "evidence" of accelerants being used when in fact they're either common to all fires, caused by the rapid cooling from water used to extinguish the fire, or are simply random occurrences completely unrelated to the fire. Yet to this day they are taught to investigators as rock solid evidence of arson.
To be fair, any experiment investigating this could only show that they are evidence of arson. ;D